Of my lady friends that I have introduced to shooting, I have always started with a 9mm, in Glock, just because they are the only pistols I have. Some of the comments from these brand new shooters like the light weight of the gun, the recoil of the 9mm round is more friendly to them, they all prefer it to a .40S&W or .45GAP, and all have shot better with a 9mm. As brand new shooters with only me as their coach, I've been impressed with how quickly a new shooter can be trained to use a pistol and consistantly hit A's. Some of my smaller female friends do have some problems racking the slides at first, but with some practice they get better.
My wife (5'6") loves my G17 and does quite well with it. One of my taller friends (6'0") can shoot the G22 well but struggles with a G37.
I'd recommend any pistol that your wife finds comfortable in her hand, is light enough and is controlable for her in all operations. 9mm is the way to go.