The stock Peugeots of that era were around 27-28 pounds, but that weight includes those steel rims, cranks, and handlebars. With aluminum components you can drop the weight to about 24-25 pounds. If you set up the bike with nice wheels and supple tires that are at least 32 mm wide, those bikes have a remarkably pleasant ride. (I have two old Peugeots, both Carbolite, so I speak from experience.) The weight won't matter (unless you need to carry it up stairs or the like!)

I know you are concerned about your wife's comfort and hope she enjoys the bike. Has she indicated that she likes the look and seating position of a Mixte? Has she ever tried one? If I were in your shoes, I'd make sure she would be happy with that class of bike first, and them you can find one that fits.