Ignore the defamation aspect. If a business is involved, I can't see that probate has anything to with it; the will itself is not being contested and nor was this person claiming as a dependant. You can always claim for losses occasioned by the wrongdoing of another, whether they are alive or dead provided you do so within time limits; indeed, you have got anyone to sue until administrators or executors are acting, in the latter case.
This is one for Barmaid or one of the solicitors on this site, such as tonywiltshire as to what step is best. Annoyingly, you are likely to have to get a solicitor, if only tell the other side to clear off! You need further and better particulars of this supposed claim, but I don't think you should be doing the asking yourself.