Bitter ex boyfriend?

Maybe your sister was really a bad gf? You never know!

There are some men (and women) out there who are extremely emotionally immature and have a "black and white" attitude towards people and life. As a result, they are stubborn, manipulative and don't have the mental capacity to see when they are mistreating others or unfairly judging them. There's nothing that can be done to change him from your sisters point of view because change is hard and it must come from within the individual. All she can do is try to ignore it which is hard and pretty useless advice but unless she wants to confront him (I wouldn't recommend that because it would give him more ammunition to be nasty) then she should just leave him be. Like a toddler having a tantrum in their play pen all you can do is leave him to it; he'll get bored eventually. Tell your sister to be be the bigger person and remain dignified throughout and that she is above his pathetic level.

She is so upset about it..

YEah maybe she will be upset, but if he is like that then is he really worth bothering about, sounds to me as if he is very insecure if he feels the need to do that. Sounds to me also that she dodged a bullet, she shouldn't waste anymore time on worrying what he says.