My mom has been banned from visiting her long-term boyfriend at a memory care home. His girls have POA. Is there anything she can do?

You should contact this Memory Care ALF and outright ask if this man has any restrictions on visitors? Plain and simple. If they say no, bring mom over to visit.

If you know for a FACT that she's been banned from visiting him and the MC will not allow her in to see him, then no, I don't think there's anything you can do about it. The POA gets to say who can and cannot visit the resident, and the facility is required to abide by those decisions.

The first thing I’d suggest if for your mother simply to walk in and go straight to his room. Some people assume ‘you’re banned’ when it’s just a threat. Any adverse reaction from staff, or positive reaction from him, may provide factual evidence if she follows up on Alva's advice.

His daughters were OK with her taking care of him, 8-10 and now it is not OK to visit? There is more to it.

If the father is still able to call her every day, won't the facility allow him to decide whether or not the girlfriend can visit?

My family had a situation like this. The girlfriend, who had her own health problems mentally and physically, complained that she wasn’t allowed to see him but it wasn’t true. So I’d make sure she’s not wanting everyone to feel sorry for her or otherwise playing the victim in order to make the daughters look bad. Not everyone in the world is playing with a full deck.

I'd call the city/county's social services and file a complaint of abuse for isolating him. An agent can check into it find out why she's being refused access to him.

Keep in mind you are only hearing her side of the story. There may be a good reason why they've decided to ban her, like financial fraud, or her presence causes him to become agitated (as I have read in other similar posts). Families that are "blended" later in life are still fraught with all sorts of emotional (and financial) complexities. Maybe they see that their own mother has been financially disadvantaged by their dad's relationship with the girlfriend... who knows, but you are only getting her side.

Since they were not married, she needs to move to protect whatever assets she shared with him. She needs to pay an attorney to help her with this. It's messy all around.

I would contact an elder care attorney if she has been with him this amount of time, but if he has such severe dementia that he cannot ask to see her the girls may get their way. This is the difficulty in these situations and your message surely does serve as a warning to others. Your Mom needs now firstly to protect any assets they held in common accounts. That is REMOVE them.
I'm so sorry. This is surely cruel to do to their Dad at the end of his life.