He instigated it and he committed an actual crime. Sexual harassment is a crime. Responding to that harassment with a tasteless slang acronym is not. Op, if I was you I'd be straight back to the school and asking them how they plan on dealing with the boy who sexually harassed your daughter and telling them legal action would be coming forthwith if they don't deal with it satisfactorily. And I wouldn't let it go. A girl was sexually harassed and they punished her for standing up to her harasser.
This in spades.
Yes, she used an unfortunate phrase, probably without knowing just how bad it was, and hereafter she will know better. But I think that if they made an example of her, they should made an example of him. If he did indeed get a lesser punishment, then I think the school picked on her as an easy target; it's easier to prove that she did something wrong; whereas to punish him would mean admitting their firewalls were not up to it.
He should have been punished at least as much; before he contacted her, she was just minding her own business. He set out to cause trouble, he started it, he deliberately wound her up, and he should have been banned from sending school emails for a week, or three, or preferably banned from the school computer system altogether, and too bad if that means he can't do his homework. After all, too bad if the girl misses three days of school, for something she didn't start? I don't "get" suspension; what's the point of disrupting someone's education like that?
I think that what the school does now is critical. It's very likely that these two will now have the knives out for each other; she because she feels hard done by, and will have lost respect for authority (I would have done at that age if I felt I'd been punished for defending myself), and he because he feels that he's been handed a licence on a plate to keep causing trouble; if he hasn't had much of a punishment, he's basically been told that he can deliberately wind up girls and get them suspended for retaliating. How long before he does it again, or starts spreading rumours in the girl's absence?