Does he want to study music? He can do a BA o BM in music and still go to law school. He doesn’t need a backup. If he doesn’t want to study music he can major in anything and do music EC’s. Credit is often given for lessons and performance.
He should submit a music supplement with solo recording/video, resume including what you wrote here, and letter of recommendation or two related to music.
You would be surprised how much a supplement can sometimes help with admissions and you don’t have to plan to major in music.