If this is the only bird that hasn't come from the breeder, and you have cured "three other hens" of IB I would actually suspect that his birds are the source, rather than this bird, as it is highly likely that they are carriers of the MG after having gone through the previous IB bout(s). The...
First step is to isolate her - this will protect her from the other birds picking at her and will also protect the other birds from contracting anything that may be causing her condition. After that - gently cleanse away all the crustiness. Saline solution intended for eye wash is good, plain...