I wouldn't recommend a <50cc machine on a rural two-laner, unless it has a wide, paved shoulder... I live on a hilly county road with a 55 MPH limit, no shoulders. It gets REALLY old having people tailgate me when I'm on the Ruckus.
There isn't really a 50cc suitable for her needs, besides the Aprilias (RS50 and SR50) - and those are spendy. Sure, a small scooter or moped will get her to her destination, but it's dangerous as hell with inattentive car drivers flying around in the same lane at twice her speed.