I think it would be best to read the specifications for the class she's actually riding in, as groom's classes can be such a range of things! I've seen everything from a costume class over a jumper course to turnout classes without actually riding...
Normally there is no special costume required. A clean, tucked in polo shirt and a clean pair of jeans will do fine. Many people wear shirts with the logo of their employer. Object of the game is to look professional, with a clean, fit horse that is prepared to show.
We have this class at the Santa Barbara National which I spectate at every year. It's right before the Hap Hansen Grand Prix class so there is a big audience already and it's quite popular. Grooms bring in the horse in hand, but also bring their grooming kits, rakes, even step stools. The judge goes over the horse, looks through the kits...it's really quite funny. Audience gets into it because most bigger barns will have a competing groom. It's nice to see the really hard working grooms get recognition! Having a nice well mannered horse helps
Make sure that the grooming kit has things that you might not think of like a show schedule, copy of the course, a bottle of water, etc. It should be as clean as possible! Oh, and the horse should be wearing a halter over the bridle with a cooler or scrim on it! Tail comb in the grooms pocket. Be creative! And don't forget a hoof pick!
I watched a grooms class at WEF recently -- There were only 3 entries -- All dressed in clean polos and jeans -- All carried backpacks instead of grooming totes (I shouldn't have been surprised by this as backpacks have replaced totes at larger shows, but this was the thing that surprised me) --
GM and Anne Kursinski were the judges --
Prize list said braiding was optional and 2 of 3 were unbraided -- The winner had the braided horse --
All horses were dressed in mesh coolers which the grooms folded back to expose the saddle when being judged -- I hadn't seen coolers used in a grooms class before -- I guess it helps the audience to identify the barns, but I think it's a nice touch to show off a combed checker pattern in a class like this --
"I never mind if an adult uses safety stirrups." GM
Love these classes! Especially when they have prize money for the grooms, they really deserve it.
To wear: jeans, clean polo or long sleeve shirt. Many have the logo of their barn on it, but a regular shirt is just fine too! I would not go for the fancy outfits usually worn in the eventing jogs; presentable but not something that would be uncomfortable to work in.
Backpack contents... same as usual but extras might include course map, schedule, small bottles of sunscreen and water, boot polish, sticky spray (lol), baby wipes, clean towels.
Horse should be SUPER clean and clipped, with hoof polish, show-sheen, the works. Bonus points for jumper braids, and unless you're going for the hunter look (braided mane and tail, etc) I would skip the quarter marks. Scrim sheet, matching bonnet and saddle pad if you have those. Tack spotless, nice leather lead rope with chain clipped to halter over bridle.
Have fun and good luck!!
She should wear workmanlike clothing and shoes. The horse should be presented as if it were going to the ring to show. Make sure to have everything you can think of in the grooming box or backpack because the judge might look and see whats in there. Turnout is key and make sure the tack is also very clean. Hope this helps and good luck.