My understanding is that there could be a deduction by the head judge, but the gymnast must first receive a warning. I would assume it would fall under the .2 incorrect attire deduction that would cover dangling earrings, nail polish, etc. I've heard of head judges giving a warning under this deduction for hair below the shoulder (which is a safety concern), but never specifically seen it for a bra strap. I think it college gym there are exceptions for skin colored bras.
Ask her coach to be sure.