^^
Man. People cheat. You can't put yourself in the middle of that. If they were married and she was having an affair, or they have kids. Then your loyalty is to your friend and he'd need to know. If SOFI tells him about his GF cheating, and he also has done the same, he'll be the one in the shit. They will make up and he'll be the evil friend the GF keeps his mate away from. Best to keep quiet. It's not worth the drama. It also depends how SOFI knows. Should he break another friends trust, if they told him?