Prudence,
I have to admit, you have often taken controversial stances on letters that I have agreed with and then confused when people thought you were wrong. Well, I now understand those that thought you wrong.
Significant weight gain in a short period is never a good thing and should be explored but I don't feel that is the core of the question here.
Eight long year ago when I met my husband, I was a lovely size 8 and he was rather chunky, but I didn't care. Since then I have gained significant weight and he doesn't care. If he ever dared to tell me to lose weight in order to enjoy sex, divorce papers would appear faster than you believe. I love him more than anything, but my weight should not be a significant factor just like I don't consider his.
The problem here is we have an egocentric older man who likes dating a younger gal and though she was large to begin with, apparently he only wants his ladies so large to enjoy. This girl is young and has a lot of life and dating ahead of her and doesn't need to be tied down with a man who can only appreciate her to such a level.
This sadly reminds me of my sister who dated a man over a decade older than her through college. When she told me sheepishly that her boyfriend was upset at her recent weight gain and less attracted to her, I was outraged. It is not a boyfriend's job to dictate how much a woman should weigh. That's a job for a prenuptial for a trophy wife.
Of course men want their women to be skinny, part of it it biological and part of it is sociological. But is a scummy thing to do to not be physically interested in someone you claim to love because of a weight gain and tell them so. That is not true love but a bit of lust and ego thrown in the mix.
Yes, maybe this gal should seek a healthier regimen of food and excercise but the easiest weight she can drop here is her boyfriend. I'd say evaluate your living condition and see how soon you can drop the fat of this guy and find someone who cares a bit more about you and not where you are packing it.