On a professional basis, there is nothing that can be done. The will is invalid, nothing can fix it, and, on the death, the rules of intestacy will direct where the estate is to go, which may or may not be in accordance with the deceased's intentions.
On a practical basis and completely off the record, without prejudice, subject to contract and contract denied- you get the drift..............
If the dad can still write his name, has his own solicitor................
Or
If the original will came to me, on a professional basis, with two signatures, how am I to know when the two signatures were endorsed?
mf