Here's what I would offer as it is what I do. She should dress in whatever outfit she owns that makes her feel sexy, attractive, desired and alluring. The color, length, material, etc. are, in fact, unimportant. If she feels it, it will translate into how she presents herself and feels when attention comes her way. Heels are nice and I usually wear them to start the evening but if she's going to dance a lot, not so good unless she's a stripper. Stay away from the cliche stuff and focus on what gets her into a sexy frame of mind. Later, if she's with a BM she wishes to gift with the privilege of asking her to wear something for him then, by all means, do so. Absent that, this is ALL about what makes her feel desired and sexy.