What happened to ND?
he choked to death on Dorian's english banger

naw, seriously, I think he got tired of being made fun of.
but who could resist? We are not the kind of folks who will let someone argue "dorian has better overall arms than Ronnie" and get away with it, now would we?
that would be no fun

.
In all seriousness, he probably got tired of trying to argue what only he could see (eg. peak Ronnie had small traps from the front, was unbalanced, unaesthetic, etc etc).
After being systematically and elegantly proven wrong for 355 pages, it was probably too much.
But I agree - he will be back.