Stribs, I am extremely sad at your loss. I too am in a similar situation. I moved to the country with two cats, with fields and copse out the back, but a road out the front, albeit a narrow windey road, it is used as a race track. One day I went to talk to a neighbour and could hear the tinkling of a bell behind me. So I walked back home and luckily he followed me.
I think you need to let the other one go and explore as it would have done. I have put Chicken wire around the garden that backs on to the road, but I would not put an electric fence. I know it is difficult but you need to let her go and hopefully she will stay on the safe side.
Good luck.