Father's partner has POA. She will not let me see him in the care home. What do I do?

I'm a little confused over the bit about her needing his money to survive. Is your father well off - well off enough to pay out of pocket for his nursing home care for the next five years? If the house is sold - if it's in dads name but she takes the proceeds- then that will be viewed as a gift making him ineligible for Medicaid for at least five years. Someone needs to explain POA responsiblites to this lady - or she could find herself charged with embezzling, fraud and elder abuse.

POAs have no legal authority to determine who can visit. As a practical matter, sometimes a care center will try to respect a request of whoever seems to be in charge (with or without a POA). But since there is no legal basis for that request, you do not have to abide by it. Simply explain your situation to the care center administrator if it comes up. Be firm but stay polite. Don't give them any reason to decide you are a trouble maker.

What is your father's impairment? For example, mobility issues, dementia, breathing issues, etc. Is he able to make decisions for himself? It may or may not be feasible for you to take dad out of the facility. Why not just visit on site at first, and see how that goes?

Just how is she keeping you out of the nursing home, is this something she has been telling you over the phone or have you actually been turned away at the door? It seems to me that a phone call to the administrator explaining your health difficulties and your desire to see him would gain you admittance. I can understand that you are not be able to take him out though, that is a commonsense precaution with many dementia residents.
I don't understand your statement about her not turning over POA, if she has POA she can't just hand it off to someone else (unless you are named as secondary and she formally resigns), if she does not but you do then she has no say in the matter.