I used to groom for someone at national level and wore wear smart (not ripped) jeans or chinos, clean Dubarrys a clean polo and, as you have a wear a hat when grooming in the ring, just a baseball cap! The first time I was really nervous of cocking up but literally no one will be looking at you so just relax.
Maybe have a few practice run through at home so you now how she likes to strip the horse off and if she wants particular areas wiped and make sure you have a small stool for a mounting block!
I agree with above! In Ireland however you aren't allowed jeans at the bigger shows but nice pants or chinos, a nice hat and some nice jodhpur boots or country boots. Definitely practice at home and enjoy it!