"You are morbidly obese."

They are now aware that their weight is a major reason for lack of success. This is an extra incentive to lose weight. In addition, it's possible they weren't even conscious of how overweight they were previously. So, they gain health benefits.

This one may not be as good as you think. Fat people are generally told repeatedly that they're fat.

The risks of being fat are generally wildly overestimated.

I've read a moderate number of accounts by fat people who found that their romantic success improved when they stopped pre-rejecting themselves.