S.A.
Just a thought.
My mother has no feeling in her chin. She often has food on her chin when she eats because she can't feel below her lip. AT times she forgets about it and forgets to wipe her face frequently, but most of the time she just remembers to dab her chin regardless.....because there is probably some food on her face. Growing up I found it very embarrassing as kids can be pretty insensitive to others dilemas. I'm ashamed that I felt that way and I can see how embarrassing it must have been for my mother and probably continues to be to this day.
I would just teach her every night at dinner to dab her face after each bite. Maybe even put a mirror on the table so she can see the food on her face and see what others see. It will make her more concientious.