Everyone agrees that B2 was thrown out of turn. But what should be done next depends on whether or not B2 was thrown contrary to the rules.
Some umpires say that it was. Following Article 16, which says that “it is the team that does not hold the point that plays,” they consider a boule played out-of-turn to be a boule thrown contrary to the rules.
Other umpires say that a boule thrown out-of-turn should not be considered to have been thrown contrary to the rules. In 2008 Petanque New Zealand (PNZ) published rules interpretations saying that a boule thrown out of turn is NOT thrown contrary to the rules. In 2012 John Degueldre, Director of Umpiring for Petanque New Zealand, followed up by issuing the following ruling.
Boules played out of turn are not considered as an infringement to the rules
[i.e. as "boules thrown contrary to the rules"]but indeed as a mistake. Players making such a mistake penalise themselves by reducing or losing the boule advantage. In conclusion, players do not incur any penalty, and boule(s) are valid and stay in place. But it is still the player or team not holding the point that must play the next boule.
The practical effect of this interpretation is that, after a boule is thrown out-of-turn, everything is left where it is, and the game carries on.
The bottom line
I think that most FIPJP umpires consider a boule thrown out-of-turn to have been thrown contrary to the rules. Note that this does not automatically mean that the boule is dead, or that the umpire declares the boule to be dead. The boule is a Category B infraction of the rules, and the offended team may apply the advantage rule described Article 24. In most, but not necessarily all, cases, the offended team will choose to declare the boule to be dead.
Personally, I favor Petanque New Zealand’s position. A boule thrown out-of-turn hurts the team that threw it and does no harm to the opposing team. No harm; no foul. (This position has been adopted by the rules of Petanque Libre.)
Multiple boules thrown out-of-turn
Players sometimes ask what should be done if Team B throws several boules (say: B2, B3, and B4) before it is discovered that B1 had the point all along. The answer is that you should treat all boules thrown out-of-turn in the same way, no matter how many of them there are.
Dealing with a forgotten boule
Players and umpires sometimes invoke the concept of a boule thrown out-of-turn when dealing with a forgotten boule. See our post on Dealing with a forgotten boule.