I am sorry that your friend is dealing with this. I can relate to the stress.
Chickens will pick on sick or injured flockmates. This is a normal chicken behaviour.
In terms of anxious plucking out of feathers, I feel for the little hen. Poor little dear. When chickens want to show they are not a threat to others they preen, it is a way of signaling that they are relaxed and not escalating the situation, that they don't want to fight. I think for sick birds they can excessively preen when they are anxious- (I have seen this in my own girls- when they are in recovery). However it is not usual to remove feathers. I would suggest your friend consults their vet to ensure there isn't a secondary infection from the surgery or possibly from another bird pecking her stiches. Pulling feathers out could be a sign of something going wrong.
If the chicken requires the crop support long term, once the hen is fully healed, she can slowly be reintroduced to the flock. This may need to be done in a neutral setting or through a barrier for a few days. Chickens are territorial and they won't want what they view as a danger reintroduced to their flock, you can't hold it against them. Your friend may also want to consider a way to help the crop support be less visually enticing to chickens. I am not sure what colour it is, but for example chickens tend to pick and peck at the colour red.. where as a black or grey may be less visually interesting.
