I'd say too late to rewind. There's been a lot of opportunities for
spells(Player A could have opted not to cast something on their second
main, for example.) In addition, and possibly more importantly, there's
been a draw step, and definite concrete information gained. A land has been
played, and (potentially, since it's not clear) another creature in the
process of casting.
Definitely a Game Rule Violation, and some investigating to make sure
Player A isn't trying some fairly bold cheating involving calling a judge
on themself for a lesser penalty, but I wouldn't rewind this.
Steven Zwanger produced this powerpoint for the Judge Conference at GP
Philadelphia regarding GRV Remedies, and it includes a couple of situations
similar to this, regarding backing up or not. Perhaps it could be the
source of some additional insight:
https://docs.google.com/file/d/0BxChLcc7HH8KQnozNmNybWlOT3c/edit?pli=1