Oh boy, so many assumptions and no facts.
Is it possible that she’s the daughter of a ger/giyores and therefore there should be no question about her Jewish identity?
Is it possible that, if she converted, she did so well before marriage and that she chose to simply not cover her hair afterwards (which, while wrong, would certainly not invalidate the conversion)?
Is it possible that it was, in fact, a sheitel, as others in this thread have pointed out?
Is it possible that the waiter was mistaken and that she was not, in fact, his wife, but a relative or business associate?
Is it possible that it was a date (and, again, the waiter was wrong) and therefore there is no obligation for her to cover her hair at all?
The Wolf