Make sure she's taking the time to relax before, during and after the show - the day before the event it was a barn tradition that the teens competing would get together for a trail ride (usually 3-5 of us) and just have a complete blast, then come back and all prepare together. Have checklists of everything and go over them with her - part of stress/nerves/meltdowns is worrying over lack of preparation IME. During the event get her to take a walk around the show grounds with her horse, or without if she's too wound up, make sure you get there in enough time so she can "regroup" and familiarize herself with the surroundings. Figure out what works best re: warmup. If she needs you or a trainer there talking her through it, or if its best for her just to do her own thing. Have another regrouping session after the show - talk over the positives, and then discuss what needs to be changed for next time.
Sorry if that was kind of rambly - but i've BTDT as a teen and hope another perspective is useful!