Yeah, sorry, but something that has the potential to humiliate and hurt someone is not a joke, its mean and nasty.
I fail to see how this would be humiliating to a grown woman unless she was going around bragging about her awesome secret admirer. This trick would be mean in middle school...y'know, the place where I presume people still write notes. But if you get a SA note as an adult, you should know enough to come to one of these conclusions: a) it's a joke b) the SA is a creep or c) the person writing the note is not mature enough to talk to her in person and subsequently not mature enough for a relationship.
I didn't get the impression that the pranksters were targeting this girl out of spite for any reason, the OP said that she was the one who had beef with the prankee and didn't mention *why* they wanted to pull the prank. Perhaps, the prankee socializes with these people and their group is one that can take a prank.