First of all you don’t know if you are his one and only confidant. You don’t know if he blabs elsewhere or that you in fact are the one and only who listens to him badmouthing his girlfriend as no one else would because they know it’s not true.

Remember your ex told his friends all kind of things about you up to the point of them advising him to not talk to you. Most decent men don’t go around badmouthing their girlfriend/wife to other women.

If he’s abused, I recommend he seek therapy or call police.

“She yells at him” means nothing. Could be yelling because he hurts her. In addition if they are in S&M set up and he is S, who really knows, he might want her to be more submissive and she’s rebelling. No one really knows

I don’t know if you should believe anyone about their relationship as no one knows what’s happening behind closed doors. But he doesn’t sound like someone I want to be friends with. I’d distance myself from him if it was me