I agree, You need to check for injuries, try to find her owner and quarantine her.
I ended up with a stray about 7-8 weeks ago. We drove around, for several hours, asking if she belonged to neighbors. We are in the country and it seemed strange that no one could/would claim the chicken. I would have been going crazy NOT knowing what happened to one of my girls..
So, I checked her over, for mites, lice, injuries.Went ahead and dusted her for them...then I kept her in a tiny coop for almost 5 weeks far away from the other chickens, then I moved her closer to the coop so they could see her, then a few days later I moved her into the run (in an area that I had divided from the big run) and put her into a crate at night, inside the large secure coop.. She would fight the other hens thru the fence and one day I came home and someone had let her into the big run and she was doing great..
If your friend has a small yard she may not be able to quaratine her far away.. So, she will need to decide if she is willing to put them together (slowly) with the chance that she may be sick and cause all three to die or become ill and weak..
My little Sassy , laid her first little cream colored egg, yesterday... Well, worth the effort.