How to convince my mother that her kindness is not always the best idea

So I live far away from my mother, and there is this one lady who isn't that mentally stable who keeps visiting my mother. The visits are not daily but they are frequent. My mother doesn't like this person but she lets her in out of her kindness. Everyone else just kicked this lady out. I don't have a problem with that and I really love what my mom is doing. I am just worried about her and her safety. The unstable lady lives alone and takes medicine for her condition. I am concerned that if one day she skips her pills and visits my mom something bad will happen Especially if she was alone I tried to talk to my mother but she is so sweet and would not kick her out for all Don't get me wrong my mother draws the line and puts limits. But she lets her in even if she was alone. I don't know how to tell her that this is not safe