I've been in security/military/law enforcement for about 12 years now. I also have given some community security/safety classes. I have a few things to point out about the security options being mentioned.
1. No device will ever be as important as education. Get a good safety/security class and stay alert. There is no piece of equipment that will help like your own mind.
2. Chemical agents. All the sprays and gasses and such are questionable. They may be better than nothing, but lack of training or understanding makes them more of a liability than an aid. I've been hit with every agent you can name, and ONLY the law enforcement strength foams affect me. Pepper spray, tear gas, canisters, you name it...I will stare you right in the face while you use them, then I'll calmly take them away from you and use them against you. I'm not that uncommon either. Everyone is affected differently by them, but many people (especially under the influence of various drugs) are hardly affected at all. You also have to be able to get the spray into play, and you have to be able to continue functioning after you yourself get some of the backspray and a host of other problems. Sprays really aren't much of anything.
3. Electrical devices. Tazers, stun guns and such are overall more effective than chemicals, WHEN YOU CAN USE THEM. You can't just walk around with it out (brandishing) so you have to have time to get it out and they're usually vairly bulky. Also, many have poor safety systems so you're risking personal injury by carrying them. There are two types, projected and contact. Projected requires distance and a lucky shot to hit the target. These suffer from thick clothing layers of protection. Contact are more powerful, but can easily be taken away from someone without extensive training.
Overall, for someone who isn't going to undergo extensive training, the best options are to learn about security. Avoid bad situations to begin with. Hiring a security professional is a wonderful alternative, just make sure you get someone good. Being seen as her security might keep away the random criminals, but it will just cause a determined individual to get more agitated. The best option in these cases is covert surveillance, leading to a strong case against the individual. They'll still be close enough to protect her if he moves, and the evidence they gather will be invaluable in court later.
Just my initial reactions, there are a lot of other factors to consider. Feel free to PM or email me for more. Good luck. (and don't waith around on this, there is no tomorrow when safety is concerned)