I'm not going to delve into the bogus "caliber wars", but "facts is facts", and the FACT is that more people are killed in the US every year with .22LR rounds than any other caliber. Walk into ANY hospital in a major city and ask th eattending ER surgeons what the most common bullet that causes lethality is, and they'll tell you--.22LR...
It's not the size of the bullet that matters--it's ALL about shot placement.
Bigger bullets buy you time in a stressful situation, because they cause a lot more damage if they hit the BG in a non-critical place.
But if we're talking flat-out lethality, a .22lr shot from a handgun is going to make someone just as dead as a 230gr. .45acp if it hits them in the heart or the brain. Period. End of discussion...
All that said, I WILL enter into the discussion of "you should carry what you are comfortable with". If your wife isn't comfortable and won't practice with anything bigger than a .22, then she should carry a .22. She's better off carrying a gun she is comfortable with and can safely and competently handle, than a gun that scares her, is hard for her to shoot, and makes her flinch.
And the fact is, a .22 (especially one like the Walther P22) is easy to carry (light and small), cheap to practice with (so she can get LOTS of practice and not break the bank) and EASY to shoot (so who cares if the first shot doesn't magically knock the BG down--she's got 10 more, and an extra 10 if she carries a spare mag). That's a LOT of bullets and I don't care who you are, getting peppered with 10-20 rounds of .22LR is going to change the attitude of even the most determined thug...
All you caliber queens need to look at the big picture. If his wife won't shoot a 9mm or a .38special, then it won't do her any good to get one--she won't practice with it, and when the SHTF she won't be able to deploy it effectively.
But if she ENJOYS shooting a .22, she will practice with it. And then if the time ever comes where she needs it, she will be confident, competent, and effective with it--and PROBABLY have tighter groups on center mass under stress than most of you "big bore elitists" have on the range...
And truth be told, once she's been shooting a .22 for a while, she will probably want to move up to something more beefy eventually anyway, like a .380 or 38special. It's all about confidence, and many women need to be confident with a gun they can easily handle before they are willing to step up to something a little more challenging (and powerful).
Have her get the gun she is comfortable with. ANYTHING is better than being completely unarmed.
Oh, and BTW, many holsters that are made for the small Glocks will fit a P22 like a glove. I have a Bianchi Special Agent (model #59) that fits my P22 like it was made for it--even the trigger guard molding is a perfect fit. Most holster manufacturers don't list models for P22's, but if you test fit them, you will find that most leather and nylon models for small Glocks fit it perfectly (My Serpa for my Glock 36, however, does NOT fit my P22). And the added advantage of leather holsters is that they look SPIFFY, and most women aren't as turned off by them for OC as kydex or polymer rigs (which look too "tacti-cool" for them)....
Carry on.