Is it privately owned? He should ask for a written copy of their staff disciplinary procedure, dispute resolution procedure and his contract or statement of employment (Scroll here to “written statement of employment”, this is required by law
https://www.charliehr.com/blog/do-all-employees-need-a-contract-of-employment/amp/)
They mightn't have these, it seems like they’ve broken GDPR rules by turning up to his private residence so not clued up on HR, but it’ll be a good indication of what you can do from your side, how sewn up they have things.
They can’t discriminate against him for anything dementia/age related, that’s against the law so if he needs someone to attend the meeting because his memory may be flawed, they have to allow it or you can be straight on them for discrimination.
If he’s genuinely done nothing wrong, the law is absolutely on his side, you have to advocate for him and make them realise he’s not going to be a push over. However, If you get the impression they really want rid of him and he’s not prepared to fight or purse a legal claim, he can ask for a payout with the agreement to leave and take it no further. They might find that preferable than the threat of legal action resulting in costs plus a payout. I believe somewhere between what he’d get as redundancy pay and 6 months wages plus unused annual leave pay is what you’d aim for.
It might not be what he wants but it’s better than being forced out by malicious claims that he can’t prove wrong and having nothing.
Call the ACAS helpline first thing if you haven’t already. They’ll give you everything you need.