What to do about this cat?

Hello,

A cat started coming to my dad's house and his girlfriend started feeding it so it never went away and we realised it was sleeping under bushes. I checked to see if it had a chip and it did. Spoke to the owner who said the cat has a catflap, bed, food etc but decided to move itself out due to not liking their dog. She said she tried to rehome it before but it went missing and she had to come and collect it as it was still registered to her chip. She brought it back to their house and it moved out.

So the current situation is: My dad doesn't want this cat. It's his home, his partner doesn't live there but she's round a lot. My dad misses the birds and is not a cat person. His partner wants this cat. She puts food out for it in a dirty wet bowl that is left outside and she lets it in occasionally (they don't have a cat flap). Whilst I stayed with my dad over Christmas, I heard the cat frequently crying to be let in but it spends most of it's time outside.

I've said we need to report the cat as abandoned so the RSPCA can rehome her. But my dad's partner is adamant the cat is happy with this current situation and the cat has "chose" them. She gets very angry at me suggesting we do something about the situation and my dad just goes along with what his partner wants.

I recently got worried because I realised the cat isn't getting the required medical treatment such as the yearly vaccinations. So I told my dad to make a decision - keep the cat and give it a proper home, bed, cat flap and medical treatment, or rehome it. He said he would speak to his partner and has now come back and said that I should pay for the vaccinations.

I am at a loss at what to do. In response to his vaccinations comment I've said I will pay for them but only on the condition the cat then moves in with me and I will give it a proper home. He's not got back to me but I know his partner won't let this happen.