I needed a top tube 50.5-51.5 cm, and got a size 50 Bianchi Eros Donna, which I love passionately---very smooth ride, fits perfectly. http://www.bianchiusa.com/06_eros_donna.html

I did also try a Specialized Dolce, which also comes in a size that would fit your wife, but liked the ride of the Bianchi much better. Also prefer the Campy shifters on the Bianchi (Dolce comes with Shimano shifters, harder for smaller hands to use).