This may be hard to determine as there are many factors at play with music acceptances.
My kid did not audition for Northwestern, however several years ago auditioned at a total of 6 schools: Curtis, Juilliard, NEC, USC, IU and UMich.
He was a strong student but without outstanding rigor. SAT was around 1420?
He was accepted to all schools above except UMich. From a music standpoint this would have been considered a safety. So our guess is that he was not accepted based on academics. But this is only a guess. He never inquired because he had such great offers at other programs.
My advice is if your musician is in the ballpark academically and musically and a highly selective academic school is in consideration, proceed with your application and auditions.
Good luck!