I'm so sorry, T.. I know it's hard. And YES, you are right that not everyone can be trusted.
Does she still have a car? You might consider selling the car and, of course, dropping the car insurance, and having her use THAT money to use cabs to get around. If there's no car to sell, hopefully she can afford cabs.
If her jewelry is expensive stuff, I'd put it in a lock box in the bank. Unless she's going somewhere nice, she probably doesn't wear it often. Maybe you could take her out once every two weeks, and run by the bank to pick it up before going to get her. Then put the jewelry back after the weekend.
If I were you, I'd transfer all her mail to a PO box and YOU handle her mail. Instead of her having a single bank account, it should jointly held with you so that if she were to get sick, you can get to the money. By having it in joint name, you can pay her bills and no one can steal her money from her.
You need to get a medical power of attorney and a financial power of attorney set up. This is SOOOO important - I can't stress how important it is. You can get a medical POA from the hospital - you want the more complicated one - not the simple one - and you and your mom really need to talk about her feelings and desires, using this document as a springboard for this discussion.
I can only imagine what she went through when her husband died, having to manage his affairs. Did she do it, or did someone else? Or did she have help? The reason I ask is that if she had someone help or if she did it herself, she is very aware of how hard it is and how much difference it can make to pre-plan. I hope she does understand this. Try to get her to choose a funeral home (perhaps the same people who took care of her husband) and sit down with them and you, and fill out all the paperwork that they will need for her when the time comes. They will open a file on her and write down all that she wants, even down to the songs she wants at her funeral. Does she have a plot already with your dad, or with her late husband? Having that set up too is so helpful.
I know this is a bit morbid. I'm doing this for my own mother, T., who is older than your mother. We did most of this for my dad, though we didn't get an obituary written, which we should have. When I got there, my mom stayed up all night and I stayed up most of the night with her, writing Daddy's obituary. It has to be done first in order to get the notice in the paper and for the death certificates.
Anyway, about the medicines. Can you put together a week's worth in little boxes with the days on them (they sell these specifically for medicines)? Would she be able to make sense of them?
Can she use a bare bones cell phone? She'd have it to call you and a taxi, or 911.
I hope these thoughts are helpful to you. I say these things because of my own experience with my dad, and what I am doing with and know that I will have to do with my own mother.
Hugs to you, T..