To be honest, it's your mate who seems like a bit of a douche here.
HE's the one who has been with her for 4 years and is denying it is anything.
You don't know what goes on before you hear any of the arguments, you have said yourself you don't know her, therefore you can't make FAIR judgements of her.
His messing around is not going to be helping her with her depression is it?
It's her that i think should be told to move on.