One of the Electra Townie models may do the trick.They have them in single speeds but also with simple internal shifting hubs or even derailleurs.
Low standover room so most folks can reach the ground with their feet while still seated.
I admire your grandma . . . heck I'm only 80 and still ride 100+ mies a week.
Can hardly wait 'til I get older!
Rudy/zonatandem