Class 2: Mont-saint-Michel by Robbins (if she's not ready for the Class 1s)
Class 1: Legende by Enesco or Badinage by Bozza
Playing a C trumpet with the rest of the section on Bb is an absolute tuning NIGHTMARE. You've got people on different partials all over the place, different tubing .. it's just a bad, bad idea. Don't do it.
That being said, it wouldn't be that hard to expand out a traditional trumpet feature piece for band and rewrite the part for C trumpet, but unless it's in F, your trumpet player is going to be playing with lots of third valves.