what would the ruling be?

[jacknine]

Playing a friendly 3 table tourney in a local pub.
Player A checks.
Player B bets.
Player A check-raises all-in.
Player B ponders and ponders and shows one of his cards to player A, hoping to get a reaction.

All hell breaks loose at the table cause some claim the hand is now dead.

What would the floorman's ruling be in a B&M? Isn't this the same sort of tactic as string-betting that is not allowed?