I hope she got her things back, working in a law firm, i know that she could have easily made a statement for theft - which would have been taken seriously!
Naww i do feel for you. But as long as the relationship is professional, I think you may have to let it slide. By all means, if you realllly are dying to scream at her suggest a " Ohhh where do i know you from?" Kind of thing, them remember it was her flatmate!
If not, i'm afraid I would just be civil to her, that way you don't need anything more to do with her :)
HTH x