I'm 5'10" also. I've got two Peugeot mixtes, in small and medium (I guess). The medium one is very similar in size to my 56 cm Trek road bike. But both fit me fine. I actually like the slightly more upright position of the smaller one. So yes, absolutely, you can find her a mixte that will work as a town bike. They are very cool, and imo, there is no substitute for their particular charm.

If I were you, I'd save up my pennies and buy that Bottechia, mount some upright bars and a Brooks B67. Your woman would be set! But if it's out of your range, I've bought 3 Peugeot mixtes and one Nishiki, all $100 each. They needed plenty of work, but you know, that builds character.