I'm so sorry to hear of your Father's recent diagnosis and the fact that his partner has decided she doesn't want to be his carer.
Yes, caring for someone with any degree of dementia can be awful, yet I wonder how she'd feel if she learned that someone would react in the same way towards her in that same situation!
After 20 years and when he really needs her, she's made her decision - she's said she's not going to care for him, therefore SHE now needs to step back from that and move back to her own home; perhaps taking up the role of occasional visiting friend, or not. Sadly he's not the one blessed with any choices, but he's certainly not the one who should be moving anywhere and out of his own home, until you're all good and ready and feel the time has come when it's absolutely necessary for his care and safety.
I agree totally with all that Pammie has said.
Social Services, even 5 years ago when I was dealing with my Mum's care, was a total nightmare. They could not have been more disorganised or under pressure. But if you're willing to be pro-active and set about making arrangements for your Father's care this will undoubtedly be of great benefit. Do speak to his GP about this, (as well as Adult Social Care in your area, and any local Dementia organisations) so you can be sure he's on their records as being in need of care; and will be assessed with regard to what sort of care may be funded and available, what sort of aids etc., (if any) an Occupational Therapist would recommend to be installed at home to make his life more comfortable, and to ensure that he will be properly looked after and not at risk.
Your own input and in regard to visiting, supervising or co-ordinating help, meals, shopping etc., for him will be invaluable and, hopefully, you will soon be able to put a plan into action involving those who are willing to make his life comfortable, and relieve the obvious concerns you will have now. You've doubtless got lots of decisions and tasks to be sorted, but I'm sure you'll get there and ensure that your Father's final years are as happy as you can make them.