A great deal depends upon how serious your wife is about learning...and keeping her practice / training up to date. Intellectually, there is no reason she cannot understand how to operate a semi-auto...but will she work with it enough that she can use it correctly under the stress and brief time frames of an actual encounter? I don't know the answer to that for you, but I DO know that lots of well-intentioned men and women just are not able to get to the range and shoot or do some situational training in the vehicle or at home, etc. as they would like to be. (Ya know, we're gonna have to get back to the range one day!)
Another factor is strength...is she strong enough to operate the weapon correctly? (You probably won't be there when she needs it for real) It does not take a whole lot of strength...but it does require some. What is she most comfortable with? I know women who, while they may prefer to actually shoot an auto, will choose to carry a revolver for its' simplicity..."point gun, pull trigger" is still done better IMO by the wheelgun than any auto...no malfunctions if the shooter "breaks the wrist", works even when pushed hard into the assailaint, no magazine to lose, no safety to remember to disengage, etc.
The auto also has some advantages as well...more ammo, usually less recoil for a given power level, faster to reload, etc. and more fun to shoot...thus more shooting will usually be done. 50 rounds from a .38 Spl airweight is a serious workout...from a Glock 26 it's just fun!
Take HER choice...not yours...just use your knowledge to guide her.
Everyone has a budget, but don't skimp on this item, nor on good ear and eye protection, nor on practice time and ammo...cut out something else to make up the difference, or go into a little debt if absolutely necessary. See Marinegrunt's post tacked to the top of the Gen. Discussion board...and imagine if that was your wife.
If you have the time and resolve, I personally like the "baby" Glocks in either 9mm or .40 if she can handle it. If she insists on something smaller/lighter, I would go with an airweight S&W .38 Special in 5-shot persuasion and use the lightest loads I could find for practice...I am NOT a fan of the various .32 or .380 caliber guns.
Good luck