I suspect my mother is being taken advantage of for money. Any advice?

My mom has a little dementia but definitely nothing she needs a power of attorney for. She has been checked out by her doctor and a neurologist and has been deemed okay. Her main problem is that she falls every now and then. In the past couple of years she has become close friends with my brother’s ex wife. This greatly upsets my brother and his current wife but I actually appreciated his ex because she takes her to doctor appointments, takes her to the pharmacy, visits her regularly and has been a god send for her I thought. My brother nor I live around her so his ex wife has been doing things for her that we can’t do and I have appreciated her until recently. Last week, I came to town and my mom and I were going to spend time together. We had planned it for a couple of weeks and we were both excited to get to spend some alone time, catching up. I called her when I was almost there and she told me that my brother’s ex said she coming too. I asked her to please ask her to give us the day together because we don’t see each other a lot. She called her back and told her I wanted to visit with her alone. She told me his ex was very upset about it. We spent a few hours together then she showed up anyway. She came to my mom’s door, looked very angry with me then completely hijacked our visit by talking with only my mom and showing her a photo album. I finally left. I mentioned to my mom later how possessive she seemed of her. She said she knows and that she gets that vibe too but that she doesn’t want to hurt her feelings because she has been so good to her, does so much for her and she needs her. The next day my mom told me that the girl put a new lock on my mom’s door while she was out. She said she came back to her apartment and noticed a new lock there. My brother’s ex called and told her she changed it for her and that you need a code to get in, which only the two of them will have. This is a retirement community and I know this is against the rules. It also means that if there is an emergency, no one who works there can get in. Interesting she changed the lock after I came to visit and without my mom’s permission. (We notified the director of the apartment community) After talking with my mom, I found out that this girl has also been bringing her buckets of ice cream and sweets (even though my mom is diabetic and currently in ketosis) and beer, telling her the alcohol will help her sleep. It also came out that my mom bought her a CAR. Yes, a car! She swears this girl paid her every penny back for the car, which I find hard to believe. She has also changed her will and put her in it. I believe she is being taken advantage of. It’s clear she does not want me alone with my mother and that she is getting money from her. Problem is, my mother is not incompetent. She has been deemed capable of making her own decisions. When I told her what I believe is happening, she argued with me that this is a very sweet girl who has a good heart and that she doesn’t want to lose her friendship. My brother and his wife are going Monday to talk to the director at the retirement home but I really don’t know that there is anything we can do legally. My mother is adamant that she changed her lock with good intentions and to “keep her safe” with a stronger lock. Interesting though, she did it after I visited and she clearly did not want me there. Any advice?