You need to provide a bit more in your question. For instance; 1) what does "proven" mean? Was there a trial? Or just an allegation by somebody?; and 2) what does "could not be authenticated" mean? And by whom?
Finally, what efforts were made to locate the witness? People move, but can still be located with varying degrees of effort and expense. Also, a Will is witnessed by two persons, so how about the second witness? And what of the lawyer or law firm that may have prepared the document?