Are the rules at home clear? Does she know what you expect of her? Are there clearcut rewards/consequences if she behaves well, or misbehaves?
Are there any priviledges she can "earn" at home for good behaviour? Or withdraw if necessary? Seems to me you need to identify what is important to her, and use that as leverage. Good behaviour means she gets to do/have those things.
Is there anything she really wants that she can work toward - for example, a certain kind of holiday/day out, clothing, etc? Sort of a teenage version of the pasta jar?
It must be so hard, and I don't have any experience or answers. I know even with dd (4.5) that I sometimes do so many special things with her anyway in our everyday lives (trips to zoo, new toy periodically, etc), that I unwittingly eliminate the concept of rewards - she gets the treats as a matter of course, instead of as a reward. So, I think I need to take my own advice!
Do you know the boy's parents, and if so, can you have a conversation with them?