What if? That would depend on the consequences of "outing" the whistleblower, and for yourself, the degree to which you took responsibility for revealing the girl's identity to the general public.

I would (and have) turned in a wife-beater to the police (he got 18 months in EOCI, extended 6 months for bad behavior), but I would not turn in a girl who outed a school administrator who tweeted disrespectful posts of a student.  I wish the two gentlemen well, and I hope they have seen not only the practical lesson here (don't get caught) but also, the moral lesson (respect all your students at the most basic level).