Father has mid to late dementia and still insists on driving. What are my options?

My 88 year old dad lives on a neighbor island with his gf who is a few months younger than he is. He drives frequently (he is from here and all of his family is here. He has been living with his gf for the past 10 years) He is in great physical health, but is in mid to late dementia and it shows. ☹️. His girlfriend is opposite, her mind is ok (showing early stages of dementia) but physically she’s a mess. She can’t walk, she shuffles and is constantly in pain from breaking her back a few years ago.

We recently found out that they were coming for Thanksgiving, so my siblings and I agreed we would disable the car he keeps here to ensure he would not drive (my dads gf got rid of their car two years ago when she recognized he could not drive anymore, but he still refused to stop driving).

Prior to their arrival, I hid the spare key in a drawer in my room and pulled the starter fuses. After picking them up at the airport and bringing them home, he immediately saw his car and wanted to know where they key were. I gently told him he couldn’t drive because his Dr said so. He was angry but, in normal mid to late dementia fashion, he forgot about it for the time being. The next day I left the house in the am to go to the gym and by the time I got back it was clear someone had gone in my room and dug thru my drawers and found the spare key because all the windows were down and it was raining. Luckily I had removed the starter fuses so it didn’t start. I went in the house and asked them who went in my room (they both said no but my dads cane was on my bed). Long story short, while I am able to hold the line with my father by distractions etc, my dads gf is insisting that she can drive and wants us to fix the car. I hve told her that if we fix the car dad will insist on driving….he’s obsessed and doesn’t understand) and she said she wouldn’t let him drive etc etc. I later that day I heard her telling him that once the car got started she would drive it out and then let him drive (because she hasn’t driven in 2 years at least and with the array of physical issues, she’s probably a worse driver than my dad!). Well it’s been several weeks and I’ve held the line on saying idk what’s wrong with it. They’ve asked my brother and anyone who will listen if they can fix the car but so far everyone has held the line which is made his gf very angry and she has threatened to take dad home if we don’t immediately fix the car. Shes beeen calling all her family and friends and telling them what a horrible person I am that I won’t fix their car etc etc and that I’m treating her like a child etc. I have repeatedly told her that if we fix the car, my dad will drive and she won’t be able to stop him. She’s still angry and doesn’t cRe, she wants the car fixed. I’ve been able to avoid it amd just ignore her when she brings it up but have to listen to her talk badly about me on the phone. I’m frustrated. They are now talking about taking the car back with them. When she told my father he had to promise not to drive he refused and said he will drive wherever and whenever he wants. She still wants us to fix the car…so she can go get a paper (we have brought them the paper each morning and taken them wherever they want to go) I am at my limit. I’m ready to put The fuses back in and give her the keys. She’s creating a monster she won’t be able to control amd he will kill someone. I’m not sure what else I can do to reason with her. Any thoughts?