Hmm. The precedent this could set would be an important one. If she just let it go, it could cause a whole boatload of problems copyright-wise, yeah?
I mean, the companies that represent her have asked fansites to close down before (restrictedsection.org comes to mind). I guesss I would agree with her action even if the snively bit of it seems out of proportion. An encyclopedia is supposed to derive information from an original work without adding additional commentary. This is where I see a divide between an encyclopedia and say, a book trying to prove that the Potterverse is an allegory with Judeo-Christian roots. An encyclopedia provides no new thesis, it merely records what JKR has already created.
Especially given the fact that she declared her plans to write an encyclopedia, what, several months ago? Ark's actions definitely seem like a violation of copyright. That being said, I don't have a clue what happens with international copyright. I think she's justified to sue.