Hi Cordgrass, welcome to the forum!!
As all the other girls said, just be yourself. You don't need to take on the man's role, but I would suggest treating her as you would treat any other girlfriend when opening doors, going first or not, or dealing with the check. He might not scrutinize everything you wear, but he will have a practiced eye. So I would make sure I am well put together with appropriate accessories in whatever style fits the occasion, whether it be casual or dressy.
Good luck, and let us know how it goes!
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