In all honesty, I think ESH

Your friend is definitely an AH - saying you look weird together because you're different sizes is ridiculous. Effectively she's saying you should gain weight so other people don't stare. Other people should mind their own business.

I totally get your frustration, but commenting on her weight in return isn't the way forwards. I think she deserved a mouthful, but you should have stuck to telling her to back off. Not easy in the heat of the moment...

My best friend has anorexia and has done for over 20 years. Do I tell her to eat more? No (though I wish she could put on weight for her own health). I'm a bit overweight at the moment. Does she tell me to eat less? Also no (though I'm trying to lose weight for my own health). If we bring up our own weight issues the other listens and is non-judgemental.