Thats a hard one man. Personally i'd probably wuss out and call him and tell him. If he gets angry at least you can prepare for a smack. Surely if he lost intrest in her it wouldnt matter? If he was dumped by her its a different story. Maybe you could you go for the 'I never knew you went out with her' Or is he a good mate? does he know you know (know what i mean ) Can you get the dads number? Ask for the girl?