PConsider counterproductive and detrimental.
counterproductive: thwarting the achievement of an intended goal; tending to defeat one's purpose Random House
I helped him cheat his way through college, which turned out to be counterproductive, since now he has no job.
detrimental: causing loss or injury; harmful; damaging (also used as a noun) Random House
Alternately, the phrase work against someone might fit the bill.
I helped him cheat his way through college, which eventually worked against him, since now he has no job.