My friend is being used by a girl he loves. How do I talk to him?

The issue is that the girl that is using him is a friend of mine, I know her better than I know him. HOWEVER, I don’t agree with what she is doing. He knows she loves someone else but he takes what he can get from her, attention and affection. It’s like watching a train wreck, I feel so bad for him. He is leaving soon, and the other guy is coming back – so yes basically switching off.

I want to let him know he needs to take his rose colored glasses off, wake up and show some self respect. I’m just not sure how to do it tactfully, the girl IS my friend too…