EXPLAINING A PROBLEM

I’ve had a problem recently. Two of my friends are going out. The boy is really in love, but I know that the girl isn’t really interested in the relationship and she flirts with other people.I’m worried and angry because I don’t know what do. If she told to him the true, I’d be happier. But if it happened, he’d be sadder.I’d really like some advice because it will help me to decide.