This might be a dumb idea, but could you just get a friend she doesn't know to dress up as Santa and visit her at home this year?
Not because she is contagious, but because you don't want the outting spoiled by other worried parents saying things, and because it will be hard when 'everyone else' gets to talk to Santa and she can't.
(Also, if she just got over chicken pox, you might not want to have her out exposed to every other germ on the planet.)