If you have someone who can help hold her, I find it easier to take a pair of scissors and trim the feathers away. Especially if the poop ball is just stuck to the ends of the feathers and not up against the skin. You can trim some of the other fluff shorter to prevent a recurrence while you're at it. If the poop is close to the skin (or against the skin), it would be risky to try to cut it out. Better to yank the feathers out than risk a cut to the rear end in that case. One of my very fluffy buff orpingtons has occasionally had this problem and I just trim the feathers back when needed.