On her neck? Right you could give her a nice warm bath, or you could maybe trim the feathers back, but if it's on her neck, that might look silly LOL. I'd give her a warm bath and rub the poop. Good Luck
If there isn't a lot, dampen a paper towel/rag with some warm water. Rub the dirty feathers with that. I don't think that a full bath would be needed for just some dirtyness on her neck.
My Buff Orpington had the same thing a few days ago, so I just took a wet hand towel and cleaned it off. The feathers were pretty dirty but it all came off.
