Honestly, having family with dementia and similar memory problems it isn't as clear cut. People really try hard to stretch the incapability for their gain sometimes. A family member tried to claim a specific thing with little monetary value that was given away was given away in a state in which our demented family member wasn't able to act properly. This led to years of court battle.

In the case that is presented here I could imagine unlawful action, but I can as easily think that OP's parent and the tenant had a different relation and were actually fine with signing stuff over. People are emotional when they get older sometimes.