Are you sure the other chickens did that? Could it be some type of predator that attacked her and she got away? Have you closely looked for claw, tooth, or talon injuries? For something that drastic to happen that quickly doesn’t sound right if it is caused by the other chickens.
I don’t know how bad that hen really is. If she is injured or if she is stripped as bare as you say, you might want to isolate her for a few days so the others can’t get to her. Give her time to recover and things to settle down. It will take a while for the feathers to grow back. If the shaft came out she will grow new feathers in a few weeks. If the feather broke and some shaft is still in there, she will not grow the feathers back until she molts.
Some hens have brittle feathers. It’s hereditary. They break so easily that a rooster mounting the hen using proper technique can cause bare spots.
It is the time of year for a molt. That does not sound like a typical molting pattern but it might be a big part of what is going on.
I don’t know what is going on. Even if you posted a picture it could be real hard to determine.