A person can have Dementia and still be able to understand a legal document, if the Dementia is in the very early stages.
I assume this legal document was created and signed in a law office. It is up to the Attorney to decide if father-in-law is still capable of signing.
If the Power of Attorney was created from an on-line document, and friends were witnesses, along with a Notary, then it might be questionable. It depends on the doctor's note as to how far advance is the Dementia, if advanced at all. If the Dementia was to a point where father-in-law couldn't understand what he was signing, then the date of POA and the date of doctor's report would come into play. You would need to see an Elder Law Attorney to see how to proceed.
I assume this legal document was created and signed in a law office. It is up to the Attorney to decide if father-in-law is still capable of signing.
If the Power of Attorney was created from an on-line document, and friends were witnesses, along with a Notary, then it might be questionable. It depends on the doctor's note as to how far advance is the Dementia, if advanced at all. If the Dementia was to a point where father-in-law couldn't understand what he was signing, then the date of POA and the date of doctor's report would come into play.
You would need to see an Elder Law Attorney to see how to proceed.