What would you do if you were me?

I think you should talk to him, ask him if he genuinely thinks it's going to last. Once you're married, it's REALLY hard to get a divorce. If they're breaking up every one/two months then the marriage thing isn't going to last. But if he is determined he's going to go ahead with it, try talking to her. You don't need to tell her you like him, unless she already knows, but just make it seem as if you're concerned about if it's the right choice. If she's just as sure its going to work, then just leave them to it. If you're lucky they'll figure it out on their own or you'll meet the one. - Hope I helped