We have only been raising hens for 3 years. Everything we have learned is by BYC or real life experience. We are NOT experts. This is my opinion.
We started out with 4 newborn pullets. They were a close and loving flock. Except, one became 'the rooster'. Eventually, 3 of the girls died from various health issues. That left the rooster hen.
We did tons of research and got two new pullets at different ages and breeds. She attacked them both, when we tried to safely introduce her. She hated them. They ended up being put to sleep for health reasons.
That left the solo girl. Everywhere, except one thread said they will die. But, I did not want to lose her. We doted on her and spent alot of time keeping her happy. Now, she sleeps through the night in her house and she free ranges a few times a day. She has plenty of sun where we live and lays regularly. She gets special treats and has a dust bath every other day. I think she will continue like this for a while. I know they don't live long alone, but she has been alone since May, and is doing fine.
Make her solo hen your buddy and give her music or special treats or yard time to keep her spirits up.