Thanks for the replies and helpfulness folks! She does get out a lot and has friends stop in - but that's not foolproof. And she does have a nice walk in shower with slip resistant floor and a grab bar.
One thing that happened a few weeks ago - she didn't hang up her phone correctly and we couldn't get through to her for one evening. When the phone was still busy the next morning we called a neighbor, who wasn't home, then a relative that had to drive 40 minutes to find out her phone was off the hook. Of course we were imagining that she had fallen, dragged herself to the phone, pulled it to the floor and passed out.
The good thing is I think we've found a solution - will have to check more into it. There is an app called Presence that turns an old iPhone or iPad into a motion sensor that will then email you. It can be used for all sorts of things - like a video camera to catch unwanted visitors or even to monitor your house while away. She has an old iPad that I think we'll set up in a room that she walks through a lot and the app will notify us that she's been through there and we will know she's OK.