Yes - a really useful reply there.
Your original supposition is correct - no child already at a Bucks GS would sit the 12+. The 11+ and 12+ are simply tests that deem the child either suitable or unsuitable for a GS education. Once inside the Bucks GS system no child would even be allowed to sit the 12+ as there would be no point in doing so!
The friend may well be moving to JHGS as a result of a transfer appeal, but he certainly hasn't taken the 12+, and the talk of "having scored more highly in the 12+ than the 11+" is obviously his way of explaining the move, saving face and all the rest of it. Maybe he is moving because of bullying, maybe the parents have forced the move on him because they are unhappy with your present GS ... who knows? If he is having to boast and fib about it, I fear he isn't a very happy bunny.
Whether a transfer appeal or a 12+ candidate takes priority would, I think, depend on the timing of the transfer appeal. Except in an urgent case (severe bullying, medical reasons or newly diagnosed special needs for example) I am guessing that both types of appeal would be heard at the same time and the decision made on the usual criteria.
An interesting question though, and one that hasn't come up before as far as I can recall - perhaps Etienne or Guest55 know the definitive answer?