Wow. Not just a fight, a women's match? I was all geared up to post (I used to help train pro fighters) but I've never been to a ladies match. I've been when they're on the lineup, but never just women.
Hmmm.... I imagine you should ask your friend. No shame there. Or call the event coordinator and ask a few, brief questions (and if it's a guy, don't expect him to be crazy about taking some time about telling a gal what to wear). Otherwise, wear something a bit daring, and glam it up a liuttle but no ball gown required, KWIM?
Say, skinny jeans that make you feel sexy, a slinky top, smooth, blown out hair and a little statement jewelry. Or be a little creative and wear some gold-gold jewelry, it's coming back and looks smart at a fight. And don't forget the wedge heels... sexy, but not in that high heels at the ballgame silly way.
And have fun. She just wants you in her corner, no matter what you look like.