How would you recommend dealing with this coworker situation?

I think you should get one or two of the other employees at your usual level together to have a conference with the supervisor about her performance. Tell her/him your complaints and have the others do the same. I know you've said that he's going to get an earful of complaints as it is, and that the supervisor and manager are already aware anyway, but it'll get his attention better if there's an organized effort to get your point across - and so he sees that it's not just you that's getting fed up. Perhaps then he or she will be motivated to have a talk with her or figure out something else to be done with her.