A friend of mine told me that he's talking to another girl who lives closer to him than his girlfriend of 3 years. He wants to see where it could go. He wants to experience at least one other woman before settling down but he doesn't see that as a reason to break up with his current girlfriend. I tried talking to him and explaining that he was cheating and that him wanting to experience someone else is the exact reason why he should break up with his girlfriend. He either needed to break up or stop talking to this other girl. He said he's gonna keep talking to her but try to keep it friendly. I don't know if I should tell his girlfriend what he's doing or just leave it alone. I've been cheated on and it sucks. I wanna save her from that but at the same time I don't know if it's right off me to do so. Any advice?

UPDATE: Turns out, he was being catfished!