Unfortunately, the lender on the debt (i.e. the creditor) had an agreeement with your boyfriend not his friend (i.e. the sublessee). Accordingly, your boyfriend is still liable to his creditor. Therefore, any legitimate collection action against him is proper. That having been said, the sublessee should be responsible for reimbursing your boyfriend (depending on the terms of their lease agreement). Therefore your boyfriend could sue the sublessee to collect (assuming that not too much time has passed since all of this occurred; otherwise any action may be barred by something called the "stature of limitations"). At this point, you should consult with a local attorney as to his specific rights under applicable state law.