Remember the needs here:
avoid attack (take a course on this- posture and attentiveness are big things);
stop attack (pepper-spray, mace/chemical & mace/pepper, tasers, handguns);
legality (New York says no to tasers and mace for civilians, check your local/state laws. Concealed pistols are illegal in some areas, or have interesting permit requirements.)
About pepper and mace sprays... Pepper-spray takes effect fast, but tends to lose effectiveness quickly. Mace takes a few seconds to really take hold, but lasts for several minutes. People who are really pumped up (drugs or bio/psycological equivalents) have been known to shrug them off, at least partially. An Air-Taser might be an idea, but with only one shot, missing would be bad.
If you decide to use a handgun, think about how much time she'll be training with it. A fast .22 to the head will stop someone, but can she reliably hit something the size of an orange, in the dark, while flustered, at 10-15 feet? If no, use something with 'spray and pray' (NOT a MAC-10), or a bigger gun (Colt M1911 or any .357 will end most problems). Remember that she might only get one shot, and she'll need the target to stop attacking, and concentrate his attention on breathing.
Next question: can she live with the risk of getting edgy and popping a musclebound, tatooed freak who pulled out his car keys and asked for the time? Or shooting at a valid attacker, and having the missed round kill a 5-year old in his home at the end of the block? Even killing someone who would have gladly done the same has real psycological impact. Much as I like the idea of having a .45 for self-defense, the risk of legal and psycological repercussions has convinced me to stay less-than-lethal for now. (Legal? Remember cases of burglars who sue homeowners for injuries, and WIN?)
A firearm is rather permanent, and while it can save your live in an emergency, it has it's drawbacks. It's a demonstrated fact that having a gun makes people both overconfident (taking the shortcut 'because I'm safe') and dangerous (Whoops- missed...).
Other options include beanbag guns (I don't if those are for sale to the public) and the Surefire method (if he's blind, he's a target.)
There was a defense-spray a few years back that included a fluorescent dye to ID the attacker. It was called DYE-ID or Dye-defender, or some such.