You can't change the outcome of their relationship, but you can be there for her in the end. Just listen to her and comfort her. I'm sure there will be someone better for her if it didn't work out with him, she just needs to jeep looking. He's out there and waiting for her. Remind her of that!
Leave it alone. If her boyfriend isn't going to break up with her and you tell her he is you could ruin what they might have had. Just be prepared to help your friend through if he dose break up with her.
Stay out of it. Best friend or not. But if he does be there for her. Reason why I say stay out of it. If they do break up and you've said some bad things about him and they happen to get back together your best friend might become your worse enemy. Be real careful. And she does have a good friend in you since you're asking for advice. Good luck for both of you.