Yep, Mr. Bogus, my thoughts exactly. If the reverse beep was working then he must have been actively backing up. Doing so with music so loud you can't hear traffic around you is, at the very least, "bad".
I don't know why but the visual I had was of a slight incline (grade) to the road where you stopped. After re-reading your post, I don't see that mentioned. I would assume had there been you would have just pulled the clutch and rolled back with him however you mentioned "peddling backward" which would make me think it was relatively flat. Yes?
Did you (Noma) ask what his reason for putting it into reverse was? I assume you both swapped paperwork?
Hope it all gets fixed up right as rain!