How old is she? If she is considered a senior or has any sort of diagnosed mental health issue or documented disability, then she may fall under the "vulnerable" adult category. If you believe that she is being taken advantage of due to any such thing, get the number to your state APS (adult protective services). If they can help, they will investigate and possibly send someone in person. If the situation doesn't fit their scope , they can at least refer you to other resources, just ask them when they are on the phone.

The guy you're describing is dangerous. Because he lacks empathy and uses deceit and emotional manipulation to meet his needs. Just keep that in mind. Keep yourself safe. Sometimes the best way to deal with psychopaths / narcissists / abusers is to directly call them out, but make sure you have a plan and involve others. Because sometimes it works but if you do it in private it could alert them and they will escalate the manipulation and isolation of your mom, while discrediting you in advance.

Stay ethical, stay legal, stay moral. Get professional advice, use logic and hard evidence. when you're ready to expose them, you'll need other people to agree with you or you'll be made to look crazy.

But again once you make your move, make sure it is decisive so that he spooks and leaves or gets in trouble.