People have used both "insidious to [someone]" and "insidious for [someone]" in published writing, but the usage is somewhat rare—at least in Google Books search results.
Instances of 'is/was/be insidious to [someone]'
From Charles Hall, "A Sermon Preached Before the Honourable the House of Commons, at St. Margaret's Westminster," (March 14, 1760):
Or however, should success still continue, through his blessing, to attend our arms; should yet more decisive conquests reduce the enemy to the acceptance of equitable terms: yet the best peace, how firmly soever it might secure our civil rights, must still be insidious to a people, not guarded by timely humiliation against the returning inundation of vice, which ease and plenty must bring along with them. The danger, in this respect, is manifestly the most likely to fall on the successful side.
From Lois Barland, Sawdust City: A History of Eau Claire, Wisconsin from the Earliest Times to 1910 (1960) [combined snippets]:
An attempt to give the credit due to the numerous individuals who assisted in preparing and conduction this celebration would be wearisome to the reader and might be insidious to some who should happen to be overlooked.
From District of Columbia Appropriations for 1963, Hearings Before the U.S. Senate Appropriations Committee (1962):
Our first experience has indicated that we have all levels of families that we are dealing with. But our experience in talking with these mothers, and so forth, indicates that they do realize, as Mr. Shea says—and some of these mothers also—and I think it is most important—have not been on assistance too long, and we are anxious to see that they do not keep on assistance so long, because dependency is insidious, it is insidious to the children, and it is demoralizing.
From Clyde Chenoweth, Our Tragic Inflation Orgy and what to Do about it: An Introduction (1978) [combined snippets]:
The "class struggle" doctrine is insidious to working people as well as to everybody else. Working people, misled by it into a united struggle to force the wage level ever higher, are unwittingly devoting their efforts to a costly, wasteful struggle to increase their own cost of living. Excessive increases in the wage level, which will eventually force the price level higher, are likely to be far more harmful to the workers themselves (or their children and grandchildren) than they are to the employers against whom they are directed.
From Robert Twombly, Frank Lloyd Wright in Spring Green, 1911–1932 (1980) [combined snippets]:
In this "progressive" period, his "radicalism" was much in vogue: "I am hungry for honest, genuine criticism," he wrote in 1907. "Praise isn't needed especially. There is enough of that, such as it is." Critical approval of his leadership in "The Prairie School" was insidious to Wright, for the plaudits neglected what he thought was unique about his own work—the expression of a philosophy of living and of a new aesthetic.
From Katherine Henderson & Joseph Mazzeo, Meanings of the Medium: Perspectives on the Art of Television (1990) [combined snippets]:
many of whom were neither educated nor churched, many of whom were not even native-born. Television, to an even greater degree than the nickelodeon, was loud, vulgar, insufficiently deferent to traditional sensibility, and hopelessly tied up with trade (this last trait as distasteful to Tories as it was insidious to revolutionaries).
From Lester Olson, Benjamin Franklin's Vision of American Community: A Study in Rhetorical Iconology (2004):
Thus, in Franklin's rhetoric, the Parliament's recent legislation not only violated the colonies' rights as granted by the Crown as a matter of historical record, but also eroded the Crown's power in relationship to the Parliament. While such an erosion of power would not have been threatening to the English, it was insidious to the British Americans in that the parliament could use such power to shift the economic burdens of the English onto the colonists without accountability.
From Justin Vicari, The Gus Van Sant Touch: A Thematic Study—Drugstore Cowboy, Milk and Beyond (2012):
That movie [Risky Business] was insidious to some for the way it made prostitution seem golden and cozy; indeed, there was so much focus on whether the women were being exploited (and of course they were) that few paid much attention to the fact that the film was really a landmark for objectifying the teen male and his sexuality. Here was a patriarch-in-training, and it was precisely this training status that made [Tom] Cruise so likable and unobjectionable.
Instances of 'is/was/be insidious for [someone]'
From Cahiers de la Femme (1993) [combined snippets]:
Assimilation, the internalization of the values of dominant cultures, entails the gradual loss of culture, language and identity. Assimilation affects all Jews to some degree. Indeed, the shrinking numbers of self-identified Jews reflects its genocidal nature. Assimilation in its various forms is insidious for all oppressed peoples, because it is at once necessary for, and destructive to, survival.
From Journal de L'Association Dentaire Canadienne, volume 60 (1994) [combined snippets]:
For irrefutable proof of how stress can be insidious for some and mobilizing for others, visit a handful of dental offices. You'll be stunned by what you see: some practitioners are overcoming the inherent stress of dentistry, while others are being overcome.
Charles Dueffer, Hide and Seek: The Search for Truth in Iraq (2009):
Clearly, by any modern standard, the Saddam regime was insidious for Iraqis and a threat to others—including the United States. The policy of containing Saddam was crumbling by 1998, never mind 2001. Something had to be done.
A different subcategory of 'is/was/be insidious for [someone]' instances
In my Google Books search results, "is/was/be insidious to [someone]" was more frequent (by a count of 8 to 3) than "is/was/be insidious for [someone]"—and oddly enough, the first two of the three "insidious for [someone]" matches were to articles published in French Canadian journals.
I should note that several additional "is/was/be insidious for [someone]" did turn up in the results—but I disqualified them because they took the form not merely of "is/was/be insidious for [someone]" but of "is/was/be insidious for [someone] to [do something]." An example of this form appears in Arthur Laffin, Swords Into Plowshares, Volume Two: A Chronology of Plowshares Disarmament Actions, 1980–2003 (2010):
These 15,000 pound bombs ["daisy cutters"] explode and spread a fine kerosene vapor into the atmosphere. A secondary explosion then ignites the fuel vapor, creating a massive pressure wave. Anyone caught in the conflagration is incinerated and the blast wave sucks out oxygen behind it, creating a vacuum that ruptures lungs. These and all other such weapons are immoral. It is insidious for President Bush (who was selected not elected) to have asked American children to donate $1.00 to help Afghan children when the U.S. was bombing their country and displacing them.
As this example vividly illustrates, "President Bush" isn't the object of the phrase is insidious in this example in anything like the way "Iraqis" is the object of the verb insidious in the earlier example reading, "the Saddam regime was insidious for Iraqis and a threat to others." Rather, the person named in the "is/was/be insidious for [someone]" construction is the agent of further action, and that further action is the thing being characterized as "insidious."
Another example of this functionally different form is the example cited by user159691 in a comment above, from Kevin Leman, What a Difference a Daddy Makes: The Lasting Imprint a Dad Leaves on (2001):
It is insidious for kids to see dads play the butt of every silly line in nightly sitcoms. Homer Simpson has replaced Ward Cleaver as America's stereotypical dad. The National Fatherhood Initiative reviewed primetime programming on the ABC, NBC, CBS, Fox, and WB networks during the late autumn of 1998. They found that fathers are central, recurring characters on only 15 of 102 network comedies or dramas, and only 4 of these programs portray the dad as both competent and caring.
At first impression, "kids" seems to be the object of "is insidious for"; but again, the actual object is the action that the kids are performing: what is insidious here (according to the author) isn't kids; it's kids seeing dads behave like doofuses is insidious here. Similarly what is insidious in the George Bush example (according to the author) isn't George Bush; it's George Bush seeking donations for Afghan citizens while bombing their country.
For constructions of the type
Technology is harmful to our environment, it makes people lazier and can be especially insidious to/for teenagers who become addicted to video games or social media.
I think that to is probably preferable to for, simply because it is more common in constructions where the "someone" named in the "is/was/be insidious to/for [someone]" phrase truly is the object of the insidious force. I would avoid using either construction if I could, however, because I think that insidious works much better in constructions where it isn't attached to a human direct object. Instead, my vote would go to version of the sentence using a word with a more natural connection to such direct objects. For example:
Technology is harmful to our environment, it makes people lazier and can be especially damaging to/for teenagers who become addicted to video games or social media.
As for which preposition I would choose in that sentence after damaging, my preference would be to because it sounds more naturally complete than for does without additional action.