I agree with the remarks, Single-Six posted. I've used different chemical sprays on MP & armed security posts over the years and can also tell you they are not always the best way to defend yourself.
Sabre Red, Fox & First Defense are good OC sprays but may not work as described due to wind, distance, the violent subject's behavior, etc.
The foam/goo type should be avoided. Many cops & security guards/corrections have deployed sticky foam OC agents only to have criminals fling gobs of it back at them.
I'd also advise your wife/OC spray owner to plan for what to do AFTER using the non lethal weapon in a critical incident. I've seen some intoxicated subjects become MORE hostile after being sprayed. I've also seen Mk III & IV size OC cans fail or clog.
I'd suggest buying a training can or a few extra cans of OC spray to learn how it works. As posted, I'd check the unit often & be aware that OC agents may expire or lose strength after 3/5 years.
Clyde
ps; I'd add that a high power LED/CREE white light with a strobe feature may be a good add-on to her self defense plan. A "tactical" pen Uzi, Benchmade, Tuff Writer, etc may help too.