Hello 7acrefarm,
The problem with chicken feed is that once it comes out as poop, if not cleaned quickly, it will dry like cement!
Soaking in epsom salt helps to remove most of it. However, there is always that one ball that just won't dissolve.
If it is attached to feathers, I - very, very, very carefully, use a pair of small pliers to crush the ball of poop. Then, soak off any remaining poo. It usually disintegrates into the water.
If the poop is attached to the vent tissue, that will complicate things. You may have to take the hen to a Vet for removal of the poop and repair of the prolapse. There is a possibility that the prolapse will need a suture or two anyways to prevent it from prolapsing again in the future.
Also, so important to keep the vent moist or lubricated to protect it. Or it will begin to become necrotic.
So sorry this happened...so distressing for you and the poor hen.
God Bless!