What a wonderful job you're doing with this dog.

Congrats, I'm sure the dog appreciates it too.
I agree with approaching the weight subject from a medical standpoint, but perhaps use humans as an example. Tell them, what if a couple came to them with an obese child who had hip problems and they told the couple that the child should lose weight to take the pressure of his hips (so as not to make things worse). But the parents said "We don't have the heart to put him on a diet, he's so happy when he gets his snacks and fried chicken and cake and McDonalds".
What would they think of the parents in that situation?
Cass.