Seperated chicken

This may be a long and sad post with spelling mistakes so I apoligise now.
A couple of weeks ago we noticed 1 of our 2 chickens had lost lots of feathers from her rear end, below her tail feathers down towards her legs, and was very raw and blooming. I was informed by someone my wife knows that we needed to seperate them as the other hen may be attracted to the blood and a canibalistic chick is not good. I seperated her from her friend during the day by letting the fit one out to roam around the garden and placing the poorly one in a cat basket at night, still within the chicken house. She has since , sorry about the term, but scabbed over and seems fine in herself. But, here come the sad bit, last weekend , not just gone but one before our neighbours dog somehow broke through the fence and got the fit chicken as she was out in the garden.
I now have 1 chicken who was safe in her run at the time but now lonely but still with a scabby rear.
I know that chickens do not like to be on their own but due to her prediciment when could I introduce new chickens to the run. It is difficult to keep them seperated in the run and let them out during the day as we both work and are worried about next doors dog.

Many thanks for any advice that you could give.

Thank you for your advice but as always it raises more questions.
She is about 18 months old and I would purchase new POL so there would be quite an age difference, we only had 2 to start with so could I just get 1 new chicken or would 2 be better?
What is purple spray?

We were thinking of 1 hybred and one pure breed, would this work?
Sorry about all the questions

we happily added 2  POL( different varity)  with no problems.  the older one was glad of the new company and showed them who was boss straight away.  it was particularly funny watching her chase them up to bed  the first few nights.