Birds do mourn over the loss of flock mates, I've seen it several times. They eventually get past it but it can take weeks, even months. As long as she is not completely alone, she will be ok. Be gentle with her, don't force her to go outside, she may stop laying and not eat much. I had a hen get nightfrights when her best friend died. So just give her a bit of extra attention, she will be ok. But definitely watch for over mating, it can kill a hen.