Speaking as a student who did his first solo in September I cannot really see how any sort of de-brief would be of any value, unless it was about the dual circuits that preceded the final solo one. I can only remember the terror I felt sitting at the hold and briefly being downwind ... the rest has gone from my memory and went very soon after I landed! I flew that circuit on so much adrenaline! I would imagine that the real value of first solo is psychological - ie yes I can safely fly this thing without an instructor sat next to me. Second solo onwards - when I was in a more relaxed state of mind - I was ready to discuss my flying and decision-making with my instructor when I got back and the debriefs from those were enormously valuable. But I think if my instructor had even tried to engage me in anything like that after my 1st solo, it just wouldn't have worked.