My ex-friend is getting really fat.
I assume you've told him this?
Yes, but unfortunately he seems to be unable to see the truth.
I think telling people the truth but in a harsh way tends to make them defensive. Maybe that's why he won't admit it. Do you think you were cruel?
I'm mean but it's gonna make him crash diet.
If that's your way of helping, I guess it works, but you might want to work on your tact.