borderline personality disorder

Well, with that in the picture it gets bit complicated. BPD is one of those disorders that makes people more likely to be in dysfunctional and even abusive relationships... but not as the victim as you might initially suspect. BPD tends to drive people to become very controlling and possessive of their significant other, and outbursts of anger/violence are somewhat common, too (and a mark of the disorder). Both of these tend to be traits of a controlling/dominating abusive partner. Their lack of ability to emotionally regulate themselves and lack of feeling of control often drives them to want to control whatever they can... like a partner... and BPD is also associated with an inability to feel responsible for their own actions (they usually feel like it's other's fault). Some common things people with this disorder do is project blame, deny responsibility, and overall exert control in whatever way they can. They commonly have impulses to do things (like start a fight) that overall harm the relationship, their SO, and even themselves. While they may dissociate and even not remember the fights or fits they had... that doesn't mean they weren't the orchestrator. While not always conscious of it, they are typically driven to have unreasonable expectations in a partner and to lash out when their (greater than usual) needs are unmet, to be blinded by their own strong and volatile emotions and thus not fully consider the emotions of others around them, and to seek to wear down the self esteem of others so that they themselves feel better. It's even been proposed that BPD causes people to have a compulsive hatred of limitations because they can't exactly distinguish reasonable boundaries (like even sexual consent) from something that's burdening them - they have this ravenous need for control and it often makes them want to cross lines of both social and legal nature just to prove that they can.

It's a really shitty disorder, and i'm not saying it's his "fault" per-se, but if it's true that the boy has BPD, it's more likely that he was taking advantage of her, and not the other way around.