Like I mentioned before, the skill to use a knife in an emergency situation is a must or the results could be less than favorable.
In response to Cordurouy's and epigram79's posts, I have to say this:
Pepper spray or any other aerosol type defensive item are by no means foolproof. In fact, they can lead to a false sense of security. For one, the effectiveness of the spray is of course based on the accuracy of the stream or mist. Wind can blow the stuff right back in your face, or deflect enough to where you're not getting the full effect, which will pi** your attacker off even more.
Another potential problem is the temperature. If it is very cold, the mist or stream will not be the same as in "normal" weather, as the particles can actually freeze before they hit their target.
Jet another thing to remember is, that those sprays are not good indefinitely. With time, they go bad. I'm not sure what the life expectancy of a can of pepper spray is, but I knoiw the Sheriff Department swaps them out every so often, used or not.
The thing I hate the worst about those sprays is, that unlike say a fire extinguisher, there is no little gauge to tell you if it is still charged or how full it is. Since you don't want to spray it just for the heck of it (considering there is very little in those keychain type containers to begin with), you're always in for a surprise when you do need it. You might press the button and nothing happens. Basically, you have to trust the thing blind. With a knife you know it's there, you know it's sharp. With a gun, well, at least you know it's loaded. It could still misfire, but you have anywhere from 2 to 10 rounds, which are not all going to be bad (unless you accidentally loaded the gun with blanks..not a good idea).
So once again, it comes down to the skill and confidence level of your spouse. If it were my wife, I'd suggest a wheelgun. No safety, no complicated chambering, just point and click. (Come to think of it, it better do more than click, more like point and BANG!!!)
If you still can't bring yourself to any decision, there are always tasers or stun guns.