Chevy makes the Suburban, maybe she has an Expedition?
Anywhere, how are you sure that all of the bulbs, wires, etc are good? Double-check that, and check the fuse box. That's what I would do. Did she take it to a dealer yet, she should do that. Some cars' taillights stop working after the alarm has been triggered, has that occurred? That could be the problem.
Good luck!