Thanks for your message. He's vulnerable in the sense that I think he is undiagnosed on the autism spectrum, just the way he is makes me think that. I have a feeling he may have 'love bombed' her, and doesn't want to lose her, so goes along with whatever she wants. One of my brothers lived with them both during lockdown and apparently she was super controlling, not letting him finish sentences etc, that was before he sold his house. His behaviour has changed since he has met her, he has given up a lot of his interests, and it's always her way or no way.
My siblings don't like her.. they get on with her, but only for my Dad's sake. They've tried talking to my Dad as well and he just brushes them off.
Thank you for responding. Unfortunately, she works from home and they both live in a flat. They both live out of the area now, about 1 hr drive away. I suppose I could 'pop in', or arrange a walk when she's working or something, I might suggest that to him actually. I think he would time it with his partner's work/lunch time and ask her along too though, which makes it super awkward because I don't then want to say 'Actually, I was wondering if my Dad could come on his own...'

They've made quite a few big purchases since my Dad sold his house, which makes me wonder whether it is what he really wants. He's had her whole flat redecorated, new flooring etc and they're just about to fit a new kitchen.. I have no idea what their arrangement is either, whether he pays her rent, or helps with bills, or even whether his name is on the mortgage (highly doubt it, as he's retired).
I feel like if they were to split up, he'd have nothing - no house, and little money.