If it's keyless remote entry with a start button as opposed to a standard ignition key, couldn't you just pull the battery out of the key fob? If the car's locked she won't be able to get in unless she pulls the hidden key out of the fob to unlock using the key cylinder on the door. Even if she gets inside, the car won't start. I'm not sure about Toyota, but on Hondas the only way to start the car with keyless and a dead fob is to hold the fob up near the start button while you are pushing the button, so that it can read the fob and determine that the key is present. I suspect they all work basically the same and I don't imagine she would know how to start the car with a dead fob. Might be worth a try.

Feeling for you on the dementia issue. Went through that for many years with the MIL. The car issue worked alright for her as she decided that she shouldn't drive anymore all on her own. She still wanted to be able to see her car every day as she had bought it all on her own and was still very proud of it. So we kept it there for her and she never once attempted to drive it. Best of luck to you. It's a tough situation to be into.