My mum passed away and my dad can’t afford to stay in the house or manage its upkeep any more.
Why do you expect him to?
According to you, it's no longer his house. The upkeep is the responsibility of the current owner - you.
When you agreed in law to be given the house, you also agreed that both or either your parents could live there until the end of their days. That caveat is still your responsibility, a fact you appear to have disregarded ethically, but would do well to wake up to legally.
If DF can't afford to live there, you owe him an alternative he CAN live in until the end of his days. And it is legally, financially, & morally your responsibility to provide that alternative, & assume any costs on doing so.
He has a disastrous relationship with money and is basically destitute.
Nice demonisation attempt, but no cigar.
It's irrelevant that he is crap with money. You presumably knew this just 10 years ago, as it's not a trait that emerges from nowhere. You still took the deal, which is an incredible one whether you end up with a flat & slightly depreciated capital, or the original house.
You sound like your DF is an inconvenience who might cost you money, & seem unable to appreciate that far from that, he had chosen to gift you with astonishing good fortune. If he is unable to live out the rest of his days as an independent elder in either this house or a cheaper flat because he needs to move to a care home, I hope you visit him & act like a grateful daughter whose dad intended her to do very well out of his estate. Sadly I suspect you will view him as an encumbrance & a personal cost instead.
Almost hope this is frothbait, & not a genuine dilemma for the guy. However shit he is at managing money.