He wanted to break up with her. He called her fat and they ended up in a fight, then broke up.
Is that exactly what was said in the letter?
He wanted to break up with her. Ended up in a fight, he said he'd tell her she was fat when she really didn't weight that much at all. Then they broke up.
That seems for the best. Partners shouldn't insult one another, especially if the insult isn't true.
Why not?
Calling someone you are supposed to care about names is cruel. You should be nice to people you are dating.