Surely if he has power of attorney, essentially she will be selling it, he will just be acting for her. If it’s a home not a rental property, she can sell it without paying tax, unless I’m missing something obvious. Loads of people sell their home to fund care home fees, it’s a perfectly normal thing to do, and loads of them will have alzheimers or whatever so have someone acting on their behalf. You need to know exactly what the (Scottish) law is in terms of how to do this right.
If it is worth tons of money or she is as rich as a rich thing, whoever inherits whatever money is left when she dies will be subject to inheritance tax, but sounds unlikely – how many Scottish ex-council houses are worth a third of a million quid?
He needs to talk to a lawyer who knows about powers of attorney.