I wouldn't get involved. You may have to take sides and loose one of them. Worse, you could loose both of them. You really can't do anything to make people stop liking each other that isn't manipulative. Your friend that wants out of the relationship needs to step up and tell his girlfriend. It's not fair of him to let her think that everything is ok. I don't know if he had asked you to help him do it but if he has, that's not appropriate either. It's not something he should ask a friend to do. All you can do is try to remain neutral once they break up which I know is going to be hard to do.
YOU DON't!! STAY THE HECK OUT OF IT!!! Its none of your business anyway. Trust me....ive been there before. When they do break up, theyre both gonna be trying to get you to pick sides! Get ready, cause when it blows, its gonna bust the roof off!
Trust me you do not want to be in the middle of their relationship and help te guy break it off with the girl because eventually it's going to backfire on you and you're going to have to choose sides and ruin the friendship between the three of you