You make a very good point. Allowing people to fail and to realise they are irredeemably unattractive is a protective measure in the long term. She will only be found by men who are questionable, harmful, or exploitative. By not allowing her to accept and make a choice based on this, FPB is condemning her to a relationship which will in all likelihood be terrible, if she finds one at all. Even if she doesn't lose weight, she will be better off knowing about her prospects and going-in informed rather than continue to float through life in ignorance straight into someone's trap. As with all things, fats only care about their immediate feelings and not their long term consequences.