Most Helpful Guy
I feel that you don't see a future between you too, and I advise you to cut all connections and making it clear to him.
So you must consider to evaluate the other relationship (him and his other girlfriend), and weather his condition is tolerable and there is hope for him to be better, or tell her because you see he doesn't hold a good future for her and you think that they should be separated.
If you know her really well there is the option where you tell her (as you know she won't freak out and ruin a possible relationship) in the hope she can help him to be a better person.
Most Helpful Girl
Think about it. If you had a boyfriend and he was doing that to some other girl would you want the girl to tell you all the things he's been doing behind your back?
If that was me hell yeah I would tell!