When did they divorce? Did they buy their house after the divorce? Power of attorney isn't relevant. The title to the house is. Can you give me the title to the house so that we can check what it says?
The power of attorney ends on death. The executor takes all decisions relating to the funeral. If there is no executor next of kin does so. If he is no longer married to her he isn't next of kin. Further if the so called will was made before they divorced it would be implied that the will had been revoked.
There is no central register of wills but there are private companies who would search for you. See nationalwillregister.co.uk for example.