I KNOW I will get skewered for saying this, but while chickens are most assuredly flock animals, a single chicken can survive and even thrive without other birds.
I have had a house hen for more then 4 years; she likes me, the cat and one of the dogs. She is not lonely or despondent. My best efforts to integrate her into a flock have failed repeatedly.
Dogs are pack animals, generally happiest with their own kind, but I rarely see anyone say it's cruel to have a single dog. Animals make their own pack. I once had a young goat who decided he liked hanging out and living with the chickens. Odd, but it made Gordy happy.
You said your girls are pets and have a gorgeous coop. Perhaps your silkie would still live a perfectly good life alone, but with your attention and care. I'd at least try that.
Best wishes for finding a workable solution, and kudos to you for taking in the street hen and getting her a friend. You clearly have a good heart.