As others said, it depends on what she is willing to train with and, especially, to use. The appropriate answer is also dependent on her specific circumstances like population density around her (urban hi-density housing? city lots? rural multi-acre lots? open farmland?) and overall character/willingness of neighbors to come running if an alarm goes off. Has she taken the temperature of her neighbors re: this guy moving in? How cohesive is her neighborhood already?
Does she live in her own home? Rent a house? Live in an apartment? That answer can restrict how much control she has over setting up security devices.
A dog, especially a loud large-sounding one (even if small size) is a good start. But as was noted, a dog also entails added responsibility which she may or not have time to do.
If she decides to go with any weapon system (knife, gun, stick, frypan, etc) training is mandatory.
One thing I haven't seen mentioned yet that is good general security is exterior motion-detect lights used either alone or coupled to an interior alarm and/or exterior siren. Lots of lights set up so that every side of her residence gets lit up sequentially whenever someone goes around the house is a good start. Mount them high enough that they cast light over a fair chunk of her exterior yard and can't easily get swatted out. Set the motion detect sensitivity to detect someone walking/moving around at the *outer* perimeter of her desired safety zone.
I prefer motion-detect lights to constant-on yard lights (mercury vapor lights) because if the exterior lights get turned on, someone or something is out there. Which area(s) light falls on tell you *where* they are. If all the exterior lights are on, assume they are everywhere (anywhere).
The "tiers" model of security is a very good and valid idea. 1st tier is your body, 2nd tier what you have on you, 3rd tier is what you can reach wherever you are, 4th tier is in the room with you, 5th tier is in the house, etc etc. I also liked the comment "condition yellow always" for her situation if she feels threatened by his domicile proximity.
HTH.
