Agree.
I am not thinking there would be an advantage to applying ED for music performance. Maybe Northwestern is an outlier, but teachers accept students based on talent and the needs of the studio. In my mind, the teacher(s) would need to hear all candidates to rank and make offers.
It sounds like your trombonist is very high level. So one can assume he would have a strong audition. But even if he applies ED, my assumption is that all candidates, ED and RD, undergrad and grad level would need to complete the audition process before decisions are made. And the down side is that he may have less opportunity for merit money if he applies ED.
Maybe the two trombonists who are currently in the studio can shed some light as to why they have their different opinions.