I think it's more likely that other(s) are doing it to her. I personally would try to figure out why they don't want to roost, and remedy that. When things become habitual to them, and comfortable, then they can be reluctant to change, so it may take some time. If you only have one roost bar...
I see the picture now, that very much looks like feather picking to me. The feathers also look very dirty, so she's probably getting dirty sitting in droppings overnight since she's not roosting. If any of your birds are over 18 months old, then it's likely they have molted, or if they are...
They sometimes molt in the dead of winter, I have one having a fairly hard molt right now also. Most birds prefer to roost, so I would try to get them to. Pictures of your set up might help. I would go out and put them on the roosts for a few days, after dark, see if you can get them to...
When I see a bare butt like that it's almost always feather picking on the roosts. How many birds in how much space? Even one dominant bird can be an issue if they don't want to share space on the roost. Do some observing after they go in to roost to see if that may be the issue. Sometimes...