good idea.... or even just a peep hole thing she can check before she opens the door?
is the back garden secure? i suggest a tall get is kept locked, if not locked already.
I assume if he starts to harass her or anything similar (lets hope it never gets that far) you will be fully supportive in her having an order put on him so he is not allowed on your property while she is there.
It seems you are friendly with neighbours... perhaps you could discuss with them that if she feels at risk, in your street, perhaps out and about with kids etc and isnt confident about making it to your house, that she is welcome to knock on their door, and wait there for a while.
It sounds horrid, but it seems you are willing to help the situation so im sure it can be resolved.
Just to add i used to nanny in a house that was directly next door to a home for teenagers in care. All of whom were sadly in care for a reason, and many were difficult and could be intimidating. We had alarms fitted on all the gates, so always knew when the gate was opened. Useful when expecting friends etc. The house was also securely alarmed and linked directly with the police station. (most alarms link to a handling system which can call police, ours was linked DIRECT to police so if ever the alarm went off (even by mistake) the police came immediatlely. It meant we had to be extra careful as they only 'allow' a small number of false call outs a year, before being downgraded to the next level, + were charged for each call out. I have to say i never had a problem in the 3 years i worked there (the home closed down about a year before I finished the job) But having ever changing, often troublesome 16 year olds who lived the other side of the fence, it was re-assuring to know, that i only had to press the emergancy button, and the police would be there ASAP (we lived a 2 min walk from main police station)