Abuse is part of the relationship? Is he in counseling and/or anger management? Is he willing to do work on that end, or vice versa?
Re-diary.
My ex found an online journal and it did NO GOOD to read. I was insanely angry and instead of taking any possible advice that may have been gleaned--I refused to listen. I can understand that if one comes across it, especially a parent trying to help their child, that's pretty hard to ignore but tread carefully. All I can advise is try to not let emotions get any more heated then need be, if she did not willingly share.
Welcome to SR and I hope the the friends and family sub-forum will be of help as will this thread. I don't know really what the "right" move is, it's so individual. I know that people can and do clean up together...but there are a lot of things that will take serious massive work. I gave an ex a chance who had hit me when using to get help, and it turns out it's more the drugs were an excuse to act in that manner and not the reason. She needs to do all she can to focus first on her own sobriety in such an early stage in my opinion.
Take care of yourself through all this too, it has to be incredibly difficult.