I would just stay out of it. If there's bad blood between you, chances are anything you say to her will be misinterpreted to throw you in a bad light. If you tell her, she might be upset and accuse you of lying, trying to spoil things for her. Then, after the prank is revealed, she'll be doubly humiliated whenever she sees you, because she knows you know what happened. (And, should she decide to complain about the prank to someone in authority, you would be the only person she could definitely connect to it. Even if your name was cleared you would probably be questioned, if the matter was pursued.)
If you have any influence with the pranksters or could alert someone in the mailroom, working "behind the scenes," that would be better, I think, and the most you should do.
Also--secret admirer pranks in grad school? I think that's incredibly juvenile--are their advanced classes not keeping them busy enough? If you want to play pranks on your friends to blow off steam, that's fine, but this sounds malicious and harassing. I wonder if there is someone like an academic advisor or guidance counselor you could talk to about this--maybe they should be aware that these things, in general, are going on in their department.