How do you convince a friend to stop overfeeding?

I think first you have to discover the why. Why is the dog being overfed? From the reason, you can then go into what needs to modified in order to suit the dogs needs in terms of eating, and what needs to be done to keep the eating in check. From their you can then devise a plan to keep the eating to a stable condition to where this problem wont arise in the future, that they can follow.