I don't know if chickens need other chickens in order to lead a full life or if you can have just one and it be happy...I'm not sure I think it can be Ok on its own.
Basically till a month or so ago, we had three chickens, all the same age, all about 5yo.
One was poorly looking for a few days then bled from its beak and had to be PTS...the vet said probably a respiratory thing or perhaps tumour etc, but she suspected lung involvement. We didn't have a PM.
Then the other day another one went - she looked ill for a day, I went in to have a look and frontline her (the other one had had some lice which we found after she died - so this was the second dose but I didn't see any on this one) and she went floppy and passed out in my hands.
She didn't recover and was dead a couple of hours later.
So we have one left. I am worried that a) she might be incubating whatever it was - if the same thing killed both the others - though in a sense this would mean she wasn't on her own long term 
and b) that she will be utterly fine and that means a life on her own with no other chickens for company.
She is in a walk in run with our two rabbits. Sometimes I let her free in the garden - which she loves.
WWYD - try and get more chickens, or just let her be alone till she goes too? It could be a few more years I know.
We weren't going to have any more but I didn't really plan for the 'one left' scenario.