If you can straighten the mixte, it might be a good bet. A one or three speed coaster brake would be nice. Upright or semi upright handlebars. If she leaves the bike outside overnight, send her a can of wd40 every year and tell her to spray the bike every month. Maybe prepay the bike shop to take care of her bike for you.

I vote for a dynohub hub over a bottle. The bottles get knocked out of alignment. Use an anti theft skewer.