That's a tough one unless we know some background on your wife. Has she fired pistols before? If so, what calibers? Did she ever have formal training?
If she's a complete newbie, I highly recommend you get her started on .22's
Once she feels confident and the fear factor is gone, bump her up to a higher caliber and I don't mean a .454 either. If you have friends, see if any have a Kel-Tec P32 or a .38 Special revolver.
I've been going to the range with a bunch of lawyers, all men. Yet, I still started them on .22's and worked em up to a 9mm slowly. In another couple of weeks, if they're ready, I introduce them to my .357 sig and .45's for defensive pistols. Good luck to you and your wife. Rarely is there a wrong pistol purchase. The only wrong pistol purchase is one that is too large and you end up leaving it at home instead of inside your holster.