Well either way, it has something to do with the chickens having enough room to preen, fluff and wiggle. And one bird is too many. Perhaps when you rehome the Mr. Cockerel, you will have room for the Pullet.
I have a 5x8x5 homemade Coop and sometimes, 6 pullets are too many and they squabble to roost.
But, no one is left outside.
But, they only are in at night locked up and free range from dawn to whenever they go in.
The rule of comfort is:
A coop is never too big but, it's always too small. Lol
Perhaps rearrange your roost.
She has to be inside. Can u just put her inside after everyone is settled at night and you put on an empty space on the roost?