Consider **counterproductive** and **detrimental**.

> counterproductive: thwarting the achievement of an intended goal; tending to defeat one's purpose [Random House](http://dictionary.reference.com/browse/counterproductive?s=t)
>
> *I helped him cheat his way through college, which eventuated in being counterproductive, since now he has no job.*
> 
> detrimental: causing loss or injury; harmful; damaging [Random
> House](http://dictionary.reference.com/browse/detrimental)

Alternately, **double-edged**, or the phrase **work against someone**, might fit the bill.

> *I helped him cheat his way through college; now he has no job. I did him a double-edged favor.*
>
>*I helped him cheat his way through college, which eventually worked
against him, since now he has no job.*