Taking BassDad's comment: <That said, many who become performance majors simply cannnot picture themselves doing anything else with their lives. They know the numbers going into the process and hope to defy the odds. There are a handful of good jobs out there (the Metropolitan Opera chorus for example) that somebody is going to get.> I have to agree.
Expanding the thought, part of this is knowing (or better yet, your d knowing) just where music fits into her life, or perhaps her very being. As individuals, we all have to come to grips with ourselves, and make choices we can (try to) live with.
From a parent's and student's perspective I'd suggest the excellent Peabody article here
Peabody Institute - Conservatory Admissions: Double Degree. There's also an excellent series from Rice here
Navigating Music Careers illustrating/exploring music career opportunities beyond performance.
Performance is an extremely competitive field, with a wealth of underutilized (and underpaid) talent all vying for a small number of select spots. If you realize this going in, have the talent, potential, drive, temperment and luck to try and make a go of it, and can live with plan B consequences, go for it.
My background involves no professional training or musical expertise. My comments are based on having a highly talented Muse driven son with a newly minted performance BM, a regional orchestra chair and grad school plans for performance.
He was aware of that "what ifs", as were we. He has to live with the outcome, or adjust accordingly. As parents, we felt confident enough to let him explore his dream. Whether he achieves it is out of our hands, but we saw no reasons for him not to try.