saying he'll get help to change.
This is the alarm bells bit for me.
I do actually believe that people can chang abusive behaviour. But there are a few provisos to that:
1 the behaviour has to be the problem rather than the perpetrator being a purposeful abuser ie they behave in abusive ways due to not having the right framework for appropriate behaviour, not because they are wankers.
2 the person being abusive has to completely recognise, accept, and take responsibility for the behaviour
3 the person being abusive has to take complete responsibility for solving the problem. They cannot expect their partner to change so as not to "trigger" them. They can't expect anyone else to behave in different ways. Their behaviour is the problem so that is the only thing that has to change.
Unfortunately, meeting these three criteria is very very difficult and so any future tense statements for me are an automatic, "I don't believe you."
So ask your friend - what has he done already to sort this problem out? Besides apologise.
DH struggled with anger and how he expressed it - because he came from a family that has a very odd view on what is/is not appropriate and he was never taught to manage his feelings or reactions. I think he probably exhibited abusive behaviour in previous relationships because he had never been taught to take responsibility for how his actions made other people feel. (That didn't make it okay but it provides context for my theory that it's about behaviour vs personality). When I told him that our relationship was over if he didn't sort it out (weeks before we got married) and he really accepted HE was the problem, he had counselling set up within 48 hours. He then did the work. And it WAS work, that took a lot of time and effort on his part. But he never shirked, he never backed down and he did it.
I am not convinced your friends BF is going to be the same.