How to help a friend in an abusive relationship?

Me and my friend aren't besties but we're closer than acquaintances haha. She, me and her boyfriend hang out. I thought he was cool...well sometimes he's a d*ck but he's cool for the most part(or so i thought he was). We don't hang out that often and I know they have their ups and downs. I mean i know they hang out all the time and do stuff together but sometimes (or maybe more often than I know or am aware of), her boyfriend yells at her or starts fights with her for no reason or for something really stupid or trivial or for some things that pisses him off but blames it on her when it's not her fault. I didn't think much of it until the other day. When we were hanging out, we were chilling at his place. i told them that I had to be home by the next day but when his girlfriend was trying to wake him up so that he could take me home, he just refused to get up. She tried to tell him a few times that he was supposed to take me home but he just simply said he was tired and didn't want to and that she should take me home. But she couldn't 'cause her steering wheel on her car stopped working. She tried to tell him that too but he just got pissed off and started yelling at her, telling her to f*ck off, and hit her. That was the first time that I've seen him treat her so violently. And she started crying and he just ignored her and didn't even apologize...because every time something like this happens, she doesn't stand up for herself and just lets it go. she seems like a very forgiving and understanding girlfriend (he has adhd and probably some other mental/psychological problems, so she tolerates everything he does) but i'm kinda starting to think that he doesn't deserve her. Psychological disorder or none, it's just flat out uncool, period. He doesn't seem to have any respect for her, whatsoever and she tolerates it. He can be selfish and immature sometimes, but this has gone too far. I may not know or understand exactly what's going on in their relationship but I don't have a good feeling about it. I don't wanna interfere in anything but I don't wanna stand by and watch him destroy her either. Even though we're not BFF's, I care about her and don't think that she deserves this. what do i do?