How do you convince a friend to stop overfeeding?

I don't understand how anyone can NOT understand the problems caused by overfeeding, especially in a society that is as fixated on weight as we are. If they refuse to accept the arguments you've been making, they probably aren't going to listen to the new ones you come up with. Is there any way that you can talk to their vet and have him or her talk to them? They might listen to a vet.