I agree with the too small bike. Like: really too small. Or rent a cruiser with flat-foot geometry (so she can put her feet directly on the ground).

Another idea I heard for kids and that could work well for adults is to take the pedals off (on a too-small bike with the saddle really low). Once they get the technique to push themselves around, lifting off their feet and keeping their balance for a few seconds, they're ready for pedals.

It goes without saying: a big piece of grass, and a very slight incline, will help.

A lot of smiling and patience and encouragement must help. I would be terrified, at my age (28) to be just learning to ride a bike when everyone else seems to have this skill.