I had about four mixtes at one point, all Japanese. Although they did come in low to mid range tubing (Hi-Ten, Champion 5, Tange 900), I found them to offer a stable and cushy ride. 26" wheels made them even better. The Nishiki Olympic Royale had forged Suntour GT dropouts, chrome fork, and the beautiful Suntour V-GT derailleur. The Nishiki Riviera also had forged dropouts and a very impressive Suntour ratcheting stem-shifter set. Both also had braze-ons for fenders.
If your wife isn't a bike connoisseur, I would also recommend the venerable Suntour components like the V-GT, Cyclone (1st/2nd gen.), Sugino fluted crankset, XC thumbies, etc. Well made, classic designs, and most importantly, they work well.
While a frame made with Columbus or Reynolds would bring in the wow factor, the humble Japanese mixte, for me, fills your two requirements: comfort and quality. I don't know if Peugeot made a U08 mixte but if they did, that would be another fantastic option. The road frame U08 rode incredibly well for me.

55cm? Riviera

57cm? Olympic Royale

53cm? Shogun 500