What the OP's girlfriend is recommending isn't really black tie. If 'most people' are dressing like what she wants, then the OP and his gf aren't attending real black tie functions. So don't worry about it. Leave the tux and all in the closet. Wear a navy solid suit (with some mohair would be nice) a white shirt, dark shiny tie, black calf oxfords and call it an evening. Then treat the evening as reconnaisance. If it's true that hardly anyone else is wearing a tux then you're cool as cabbage. If not, then point this fact out to your girlfriend and head to the bar. Wear your tux the next time, and wear it the way you want it.
Now that's some damned good advice.