Who could I contact if I know my elderly neighbor is being taken advantage of?

I just moved into a new area back in December, the first person to ever say Hi to me was a nice old man across the hallway. It took us a few months but we eventually became friends. He took a hard fall last winter and I started going grocery shopping for him, helping him out with things like that. We used his car for a little while. I got into an accident in it and paid for a tow truck and damagaes. Well recently he had a "friend " move in with him. This lady is like 40 years old and homeless. She has been redecorating his whole apartment to HER LIKING. Yes his apartment needed a deep clean but she has moved things around and even threw things away. She's been using his car and she dosen't have any money to get it fixed if something happens. She had a flat tire and the old man had to pay for it. She's been taking his car for 24 hours not caring what he has to do or needs to go to. He needs a full-time caregiver and she's trying to be that but SHE'S NOT EVEN THERE AT NIGHT WHEN HE FALLS. He has to call the ambulance. I am recently 9 months pregnant and can't help him like I was and we see how she moved in and took advantage of him. I don't know who to call or what to do. He has no family, barely any friends. I'm just at my wits' end with it because I told him she's taking advantage of him and he still lets her do it. He used to be a drug addict and stopped due to health issues and she is a drug addict and I feel like she's feeding him drugs. I have no evidence but things have changed in the past 2 months to make me believe so. Who could I call to get them to look into this? He has nurses that come see him during the day and they all feel the same way. I just don't want to see him down and out and get evicted because of this lady. Please give me some advice.