i take it he's a teenager?

it's tough, but she has to shield her heart from hating him. there's usually an underlying cause, and if he feels like his family hates him (which i suspect he already does) it'll only get worse. that kind of behaviour is usually a shield to make up for /cover some flaw he sees: low self esteem, rejection, need for attention, who knows? but hating him will def not help. she might want to think about ways in which he is treated differently from others, (including treatment as a result of his bad behaviour), and correct that deficit.

is her parents aware of the abuse?

There is alway two side to a story. I believe your friend, or some family member is doing something
that is probably making him put on that behaviour, or he is going through some emotional problems.