It's not something that is contagious, you don't really need to worry about the others. I can happen to any hen, but it's not spread between them. Only you can decide when it's time. The truth is that from survival instinct they hide illness very well, so if they are acting sick, you can be assured that they are very sick. Since this is not something that is really recoverable from, it's usually a kindness to not let it go on once they are obviously miserable. But when that time is, is your call. I will attach an article which discusses various methods, I euthanize the same way as the article, for many of the same reasons. It's also possible to have a vet do it if that is accessible for you. Or if you have a neighbor/friend who also keeps chickens they may be willing to do for you, or assist you. It's not something that anyone likes to have to do, but sadly it's something that eventually everyone has to, if they keep chickens.
https://www.muranochickenfarm.com/2017/06/the-easiest-way-to-cull-chicken.html
