My friend is cheating on his girlfriend. I don't want to get involved, but shouldn't his gf know?

My guy friend has been dating his gf for a couple of years now, but he's always cheating on her. When she finds out about these girls, he tells her these girls are turning against him. I feel sorry for this girl because she's in love with him but she doesn't know what she's really in love with. He's tried to get all my friends to hookup with him and he has succeeded with some but his gf just either is really gullible or she cares too much to give him up. I don't want to get involved and loose a bunch of friends but I think one of my friends might talk to this poor girl. Any good advice?