Those of you recommending bear and wasp spray mean well I know, but keep in mind part of the item being useful is it's size and weight. Those sprays usually are more bulky - that can dissuade someone from carrying if they plan on traveling light, or it simply can't fit it into the purse of choice. It can't help if it's not with her.
Get the right tool for the job, but you do have to research them as they are definitely not all created equal. I recommend looking at Cold Steel and Foxlabs products. Both will make an assailant hate life in short order. I like the Cold Steel sprays for women myself. It's a really nasty foamy consistency. They also include black pepper extract for that extra punch to the airways, makes you inhale more nasty stuff. Many sizes, and they have great quality seals (very important). Anyone who says they are useless doesn't know what they are talking about. Get back to us when you've had one of the above mentioned brands shot in your face.
Get a few inert trainers too. Get her used to how they fire, then have her practice both a quick grab and spray, as well as the subtle sneaky draw and fire. Doing that with real spray can ruin your day if you mess up.
I'd pair it with something to harden the hand as well. Something like
http://www.amazon.com/Safety-Cat-Wom.../dp/B004LUXICC
Proximity to the threat could negate the ability use a distance weapons, so don't leave her with just manicured nails to get her out of a bad spot.
The most important thing for young women is not what brand of spray they carry, or if they opt for baton instead of a stungun. Situational awareness and not being naive counts far more. No amount of self defense gear is going to help a girl if she is in the habit of hanging out with strangers, getting shitfaced in public, and/or is just plain oblivious to everything happening outside of 50ft from her location. Pretty much all of the girls I knew in college were guilty on that last one.
My baby sister is quite the knock out. She was a model for a time. I had to take her aside and spell out things that invite the attention of predators. Walking around in public with earbuds in is a no no. You don't go out to party at night alone. You always get your own drinks. Always have your own transportation plan for getting home, never rely on people you hardly know for anything. Don't be afraid to look people in the eye when they approach. Looking timid and meek only encourages assholes and bullies. Made her into a people watcher before I equipped her with self-defense tools.
