instead of giving them a non-rigorous reason to not hate themselves, don't give a reason

I think there are more options than just a more realistic assessment of how well someone is doing at something. For example if someone thinks he/she is fat, one could point out that there is more to attractiveness than body weight or that there is more to a person than physical attractiveness. If the person is not really fat, pointing that out would be helpful, too, but while doing so the believe that body weight is important shouldn't be reinforced.

I agree that just saying "stop it" is hardly helpful and it was not my intention claim that it is.