Actually, I think this was the FIRST time that F Cup-chan was selfless and not possessive. I don't see much of anything she's ever done to be truly out of "love" for Watari-kun.
F Cup-chan has always been, "me, me, me!", whereas Satsuki has been there for Watari, helping him, and not asking for anything in return. All of that seems to have gone unnoticed by Watari.
I think Satsuki was probably leaving, because she felt that F Cup-chan had won, and there was no point in staying and trying to win Watari-kun's heart, but if F Cup-chan is going to give her a chance like this, she'd be a fool not to take it.