You can try to send the letter to her requesting the plumbers' fees. Also you can have hr get her own plumber to inspect to prove there is a leak. What I'm confused about is why she is not making the claim against the landlord (unless you each actually own your units.) It could be harassment if thee really is no leak. Just put it back in her ballpark to prove it to you. Good luck.
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I agree with Counsel Allen that it would be helpful to know if you own a condominium (and there would be a Homeowner's Association) or if you are both renters (and there would be a Landlord) to determine who is in the mix. It is also not set forth what she said in her letter other than there was a leak. If your plumbers found that there was no leak and your downstairs neighbor made specific representations that were false (and you relied by hiring plumbers) then I would request the money back and (not that you want to hear this) you may wish to pursue hr in small claims for the fraud she perpetrated on you.