My 70-plus mother gets offended when a restaurant server asks a table composed entirely of women, 'What will you guys have?' She very loudly points out that there are no guys present, only ladies; when the server asks what the ladies would like, she will then place her order. Is she being overly sensitive or is she justified in requesting that she not be referred to as a 'guy'?
It depends where you've taken her. If it's a casual place that's geared to the subgeriatric set, yes, your mother's probably being oversensitive, though not by much: A good server should be able to size up a table and tailor his tone accordingly, even if he's got neck tattoos and a faux-hemian wildman beard.
If the restaurant's more formal – linens, crystal, widely spaced tables, or fine food at very least – your mother has every right to be referred to as something better than a "guy." All that said, formality doesn't always come easy, even at otherwise accomplished restaurants. At one very expensive and moderately formal place in which I ate last year, the server, hoping to take our table's order, announced, "Shall we start with the lady?" The only problem: There were two of them.