Most people are pretty bad at taking criticism even if it is constructive.
If you decide to help out, just kinda drop hints like, man if I did as much cardio as you I don't think I could pack on any muscle as all. or I use to do alot of cardio but I couldn't pack on any muscle when I did so much. hows it working for you?
either way, he won't stop immediatly, but when he notices that his lifts never seem to go up, it will be in the back of his mind and he may cut down.
We've got a guy like that in our gym. Barely ever lifts but he does 10-15 hours of cardio a week! Everyone teases him about it. but he just doesn't ever stop. Once in while people tell him hes really fucking up his system. This guy drinks a real lot then runs off all the beer calories. If hes hurting like legs cramps or sore knees ect.. his idea of a cure is to run it off. He often jokes about how week he is and its sad because its true, spends so much time in the gym just to stay nuetral with his beer gut.