Why not a mixte, assuming that the rider isn't planning on racing? No real disadvantages, and at least one big advantage: the headtube is always substantially longer than on the equivalent diamond-frame size. I'm surprised that no one ever seems to mention that in mixte discussions. The taller headtube is a big benefit for women with long legs and short torso and arms.

By the way, the OP might be well advised to PM big chainring on that yellow Bottechia mixte. Steel rims, steel cranks, Simplex gearset, entry-level bike-boom bike; it's beautiful, but its value in the marketplace is not particularly high.