She still drives and can carry on a reasonable conversation but when she sits down, particularly in a comfortable chair (or on the sofa) she falls asleep and falls over. This happens all the time. She lives alone and we are concerned about her when no one is there with her. She has a pacemaker.
Now, I am concerned about your Mom falling actually out of a chair. My Mom would doze off in her wing back chair and lean to either side, but the wings and arms on the chair would catch her.
Thus, if you Mom is sitting in a chair with no arms, I would move that chair elsewhere to be safe.
You may also want a make an appointment with your Mom's primary doctor to see if there is a sleep-balance issue.
Let us know what you find out.
She should see her cardiologist as often as they recommend.
My dad had this going on and it was because he had so much water retention and his generator was running down.
I would get her to her cardiologist and tell them that you want her hardware checked. Be sure and tell them that she is falling asleep and falling over everytime she sits down, this should get her seen quickly.
Best of luck, it is scary to see this happen.