I like a lot of ideas in this thread and cannot think of much of anything that hasn't been said. I also disagree with some ideas.
I think getting a dog is a great idea. The cost of training it is well worth what is at stake.
Along the lines of training, I think that self defense training is a must. Also, I think a gun that she is both comfortable using and trained to use is a must. I disagree that having guns (or any weapons for that matter) laying about is a good idea; an attacker could easily use the weapon against their target. A gun and a knife should be carried at all times if possible, and kept hidden, yet accessible when it is not an option to be wearing them.
As far as what Busse is best, I think it is the one that your sister is most comfortable with using and is trained with. If possible, a D-Guard or lanyard should be used at all times to insure that she is the one with the weapon. A Boss Jack or Active Duty could be perfect for smaller hands. I keep my .22" ASH nearby at all times at my apartment. Even though I don't currently live in a rough neighborhood, I have in the past, so I am used to keep weapons around. That leads me to my next point, securing the apartment.
Long story short, I once had a (female) stalker who would break into my apartment and didn't understand the word NO.
I spoke to my landlord/manager and had them install high security locks to my back door, which leads right into my bedroom. It wouldn't hurt to talk to the landlord to see if this could be done at her place.
Mace is another great option as a last resort.
I have experience dealing with a sexual predator who abused a woman that I love (ex fiancé) when she was just a child. I won't tell the full story here, because this thread isn't about me, but I would be happy to tell it to you privately, Lexi, and offer any advice I can (the man was her father and yes I did meet him. He's alive, but I doubt he ever talked to his daughter again.) The difference in this situation, and the reason why I bring it up is that your sister has the chance to defend herself, should this twisted sonofab**** tries something. I truly hope that he won't, but it's best to stay vigilant. Prepare for the worst and hope for the best.
I wish you and your sister the best from the bottom of my heart. I hope that the predator mentioned gets what he deserves and that he does not try to attack anyone again. Sexual predators are the scum of the earth and it makes me sick to know that they exist.