I am a regular who has name changed so I am not identified if extended family spots this thread, I am posting on someone elses behalf for advice of what to do next but think I'll explains details in first person for ease of clarity.
Apologies for the length of what is to follow and have changed a few identifying details...
Edith & Alf married when her sons were young adults and they didn't like step dad they found him cruel and unpleasant. Edith made no secret years that she didn't like Alf for the last 20 years of her life and that she had made a will that gave her share of everything to her sons - very common knowledge amongst wider family, always said that all her savings were for the sons and grandchildren.
Around 5 years before her death around the age of 70 she passed on her main savings passing book containing £100k in her sole name to one of her sons and told him it was for safe keeping as she didn't want Alf to know about the money. A couple of years later she developed dementia and this son and Alf were giving Power of Attorney.
Shortly after this time her wayward son turned up out of the blue and managed to steal £6k from a different savings account - he took all of the passports, passbooks everything and forgered her signature on a cheque etc. They never went to the police. Obviously he never saw the £100k passbook.
She has now died. Before the funeral Alf was on the phone saying there were no savings on around £2k and saying please don't make him homeless etc. Since the funeral he has bought a new car, a new TV and several other significantly expensive gadgets.
In her will the her half of the house is split between the sons Alf gets half as they were tennants in common. Any joint savings were his, any sole savings were for her sons but there were savings in her sole name.
The wife of the son with power of attorney went to the building society with the passbook detailing the £100k and explained who she was and asked if the money was still in the account. They could only tell her, due to confidentiality, that the account had either been closed or moved.
Due to a few other "odd" things said by Alf and NO evidence of Edith having spent ANY of the £100k the son with power of attorney is very suspicious that Alf could have used his power of attorney to transfer the savings into a joint account - presumably they had to notify the building society about the missing passbooks etc and he then could have found out about the £100k account.
Alf has suddenly suggested selling the house and giving the sons their money now. Unless Alf has signicant savings his share of the house is not enough for him to buy a retirement appartment, but if he did have a nother £50k or so he would...
Alf's solicitor who is handling the will wants to come and visit son with power of attorney on Saturday and so suddenly Alf does having not ever asked to see him since Edith died.
What should the son do????
I have suggested he should go the build society in person on in writing with proof of power of attorney and ask about what happened to the money in the savings account as the passbook was given to him for "safe keeping"
Should the tell the solicitor who has handled the will about this, should they be instructing their own solicitor or what 
Any advice with what is the best thing to do?
I can honestly say Edith made it very very very common knowledge that she did not even like Alf let alone love him and there is no way she would have signed over the money to him had she been of sound mind 
If it was a small amount of money it wouldn't be an issue, similarly if their had been evidence of her having spent money in the last 5 years of her life but there isn't it has just "vanished"
I have warned them if Alf has done something then they will have to report it to the police as it is a serious matter.