well although you agree that her desk is indeed messy you could be more supportive of her, maybe she is not very tidy, it isnt illegal, could you help her sort herself out? most people sit on the fence because they dont want to get on the wrong side of the accusers and face a confrontation.
the people that befriend bullies the "cronies" are a good example of people that are too scared to get involved.
I would personally try to help the person and tell the people b!tching to concentrate on their own tasks, Im sure they are not perfect.