I used to support the idea that one shouldn't molly coddle tubbies because it's not a healthy condition, so one ought to take some action to dispel the beliefs in some folks that being obese is just dandy. The question, though, was what action.
That depends on the person and the nature of the relationship. Full out stigmatizing the fatties is really only best reserved for drill sargeants and or people sharing a life boat/desert island. The former because, well, hell, that's how boot camp works- break down the old person and rebuild the new.
Somewhat less severe stigmatizing, or rather just teasing, is fine between male friends. Refering to your male friend as "fatboy" is normal, and if he starts crying or binge eating, then you can stigmatize him for being a pussy.
Female friends? Nope. Can't do it. In this case, you ought to find a better, more positive way to get them to lose weight. This is a major problem when dealing with a tubby daughter. My kid is overweight and I refuse to even acknowlege it. Why? Because I'm also responsible for her self esteem and hitting a 13 year old on her weight is way to, uh, heavy. So, no stigmatization, and in fact, the dad's always got to make the kid feel good about themselves (well, not in terms of behavior-- that's where you can lay it on them hard and heavy).
So, that leaves these options: being a good example in diet and exercise; getting the kid to participate and try to enforce good eating and activity habits; putting them into enough activities so they aren't sitting around with nothing to do but watch the tube or surf the net.
But stigmatizing a daughter will just end up fucking her up in many fundamental ways that can be far worse than the health effects of obesity.