Can I get guardianship of my Father who has dementia if he has already made someone else his POA?

Yes, you can file for guardianship. Are there medical professionals willing to tell a court that Dad is incompetent to make his own decisions?

Consider, though, that Dad has already expressed his trust in this woman, by living with her, and by formally naming her to be the person who makes decisions for him if he is incapacitated. Do you have good grounds to go against what Dad has decided?

What would you do with him instead of bringing him to his home after hospitalization? Place him in a care center? If he is seriously sink and also has dementia that may be the decision his significant other will come to eventually. Perhaps she is not ready to face that yet.

What is she doing that demonstrates that she is "unfit"? Does Dad go without meals, not get help getting clean, not take his medications on time? How would you explain to a court how living with her is putting him at risk?

Do some reading about how to go about obtaining guardianship, what it costs, and what your responsibilities would be if you obtained it.

Are there other ways you could help ensure Dad is well taken care of, without resorting to that legal action?

And that legal action will be very expensive! But, how far is Dad into dementia? Does he understand the purpose of the POA? If he does, would he want to meet with his attorney to change it? Dad only needs to convince his attorney that he understands "in that moment" and POA could be changed that way. Does he have a successor in case the woman predeceases him? If you think Dad is capable of executing new POA's then it may work to suggest to him that it is time to have his docments reviewed by his attorney.

What is putting your father in hospital?

How old is the girlfriend? What are her health problems? How long have they been living together?

You could apply for guardianship, but that's not what your father wanted which puts you at a disadvantage from the get-go. I'd try alternatives first, if I were in your shoes.

For example, the next time he is in hospital or anyway receiving active treatment, use the opportunity to involve social services and get their home environment assessed - this would run a check on whether both of them are getting the care and support they need.

Have you had any discussions with the girlfriend about how she is managing, or are you not on speaking terms?