You really need to speak to a solicitor, he's going to need one anyway for any house sale to go through. They should, as matter of course, assess his mental capacity to make his own decisions and if they are in the slightest doubt, they can refuse to act.
If you suspect that the partner is gold digging, it may be worth asking what he intends to do with the money from the sale.
Keeping Dementia Sufferer Occupied


