I heard someone use a phrase that I thought perfectly captures this but can't remember it. It was used in a situation where what someone did was clearly wrong, but they pointed out that everyone else was doing the same thing.
The bandwagon fallacy assumes something is true (or right, or good) because other people agree with it.
That's the last one of 15 Logical Fallacies You Should Know Before Getting Into a Debate.
And from Bandwagon Fallacy Examples on yourdictionary.com...
The bandwagon fallacy~ describes believing something is true or acceptable only because it is popular. The fallacy is also known as "jumping on the bandwagon" or argumentum ad populum ("appeal to the people").
In the common vernacular, people often mock this particular fallacy with variations on...
If the behavior is specifically self-destructive, there’s the expression “if everyone jumped off a cliff, would you do it too?”. This can often be found with slightly different wording, such as “your friends” instead of “everyone” and “a bridge” as the medium to jump off of.
“If everyone jumped off a cliff, would you jump too”? is parents’ way to tell their children that stupid and dangerous behavior should not be copied thoughtlessly. —Medium
This logical fallacy is also called
Appeal to popularity
Kid: Why won't you drive me to school? All the other parents drive their kids to school. ...
Parent: That was an Aristotelian enthymeme, dear! ...
Meantime, if you feel especially obnoxious, name the fallacy: the appeal to popularity, which legitimizes your choice by claiming that others have chosen it. My children would rather suffer an old-fashioned caning than hear me label their fallacies.
(Source: Heinrichs, Jay, Thank you for arguing, pp. 140-141)