I would seriously be concerned about folks giving advice wrt various devices which may or may not be legal in your jurisdiction, (not to mention the dicey concept of ‘self defense’ which doesn’t always mean what folks think it does, in various jurisidictions).
Before I bougth such a device, I would make damn certain that the device was legal in my jursidiction (and no, not just by asking the seller), and under what circumstances they could be utilized legally.
Sure, if you’re going to go and be all reasonable and stuff…
I self-identify as a pacifist, too (no! you all say, shocked 
), and while I’m not going to go around beating anyone else up, that doesn’t mean I won’t protect myself. Her best defences, in my opinion, are changing any numbers he might have to contact her, and letting all her friends, family, co-workers, and school know that he hit her and he is not to be given any chance to contact her.
She also needs to be prepared if he does contact her again. She can prepare what she is going to say to him in advance, so she doesn’t get caught off guard. She must prepare herself for him to be as sweet as he can possibly be, with crying, emotional breakdowns, etc. Remember for her to use lots of “I” statements, not accusing him of anything - “I don’t want to see you any more - it’s not working for me.”
Is she seeing a counsellor about this at all? It is very common for people who are abused to blame themselves for it - for not seeing it sooner, for sticking around for it, for lots of things.
I don’t think she should make any statements at all. Let her silence, and better yet, departure, be her statement.
When I first got to college, my brother got me this book called Attack Proof. It suggests that if you walk through dim-litted areas, and you feel unsafe, to carry a ballpoint pen with you. If anyone attacks you, you could use it to stab the person in the arm and get away. It doesn’t cause much damage, but it inflicts a great deal of shock to the attacker, plus it’s innocuous enough to carry around. To me, it’s more reliable than having to unlock the safety of pepper spray and then having to aim and shoot.
I know this sounds horrible and extremely violent, but on my campus, there are a number of assaults that happen at night and I’d rather be ready for things like that.
Mr Bus Guy, for what it’s worth, see if you can get hold of a book called 'The Gift Of Fear" by Gavin de Becker. I found it very helpful. He also has a website.