Protein deficiency is actually extremely uncommon, which I have to explain to people over and over again. Even vegans get enough protein. Even people who eat pure junk get enough protein. You would have to be virtually starving in a third world country to have a protein deficiency, especially in our super-fortified, post-WWII world.
That said, unfortunately, there is no answer. Even when you're still healthy years from now, people will be convinced that this will cause you to drop dead any minute now. People don't see because they don't want to see. I wish I had something better to say than that.
Eventually, though, she'll keep her opinion to herself, for the most part. That's the upside. I recommend venting to your veg*n friends both online and offline, and, as others have suggested, ask them silly questions and laugh. : )