Unfortunately, there is no system of registration in England and Wales for Wills other than a voluntary one (which is very little used). You cannot apply for the Grant of Letters of Administration until his widow is "cleared off" (ie someone with a greater right than you has renounced their right). I would suggest you lodge a caveat against her getting a Grant of Letters of Administration IMMEDIATELY. Then go about finding (through advertisements or writing to his solicitors) a copy of the original Will.
I disagree that this is not worth doing anything with, I consider that IF it is proved that he did revoke the Will, the wife stands to inherit substantially. If she has spent some of his money while purporting to act under the Power of Attorney (which only needed to be registered once he lost capacity), this could be taken into account in the intestacy, but to do so, you will need to prove that the "gifts" she made were as a result of undue influence.
What Themas has said is really important too. Let the banks/building societies etc know he has died. Whilst the PoA died with him, if the banks do not know of the death there is nothing to stop her continuing to act under it, albeit illegally. Contact them ASAP.
I think you need legal advice from a specialist firm (preferably a STEP or ACTAPS) member. And pronto.