Mackay is spot on. You invited Mary to a confrontation and she declined. In that same vein we don't have to go to all of the fights we are invited to attend. I agree that you should address the work issues proactively with her, your manger and hers, and document them so there is no suprise in your performance review: "Failed to serve Mary adequately."
I don't know how far your manager is willing to support you, or if you can have this discussion "unofficially" with him, but from your manager to her manager he may want to mention that Mary and her manager are indeed treading on a "discrimnination" issue. Gosh he's a good employee (you), does his job well, I don't think he's like this, but he may/could turn around and sue us for a hostile work environment if people don't lay off his cultural heritage. (It's got to be protected 10x in California, at least that's my perception on the right coast.) NO manager wants to deal with that crap. It's real important to get the work part nailed down first, and perhaps your boss wouldn't do this or you don't want to talk to him about that aspect. It's just a thought

You are indeed at the wrong end of the barrel, good luck in turning it elsewhere.