Theater, my dear, theater.
Smile at her really hard whenever you see her. If she starts to complain, smile again and say "Woops, gotta go "make-up task" right now!"

There was a really always cranky lady where I used to work. She'd scowl past my desk many times a day, and was generally unpleasant to deal with. I finally decided on the above technique. I'd smile so hard anyone who KNEW me would have either run scared or at least known it was fake, but ... after doing this for months, she finally stopped one day, and said "You have such a nice smile," and smiled herself. As people around me practically fainted dead away (we all thought her "smile muscles had atrophied from lack of use) I kept my grin, muttered something about crediting my orthodontist and parents for paying the bills, and she continued on her way, still smiling. We kinda became "acquaintances" after that, in that she'd kinda smile when she walked past ever after, and treated my co-workers a smidge nicer, too.

Worth a shot! And at least you'd be exercising YOUR smile muscles, while thinking whatever evil thoughts at her you'd like!