The Law says
Law 9B1 said:
{a} The director should be summoned at once when attention is drawn to an irregularity.
{b} Any player, including dummy, may summon the director after attention has been drawn to an irregularity.
{c} Summoning the director does not cause a player to forfeit any rights to which he might otherwise be entitled.
{d} the fact that a player draws attention to an irregularity committed by his side does not affect the rights of the opponents.
The laws say, of "should", that failure to do what one "should" do is an infraction, albeit one which will seldom draw a procedural penalty. So. The director should be summoned. Any player may do so. Regarding "may" the laws say that failure to do what one "may" do "is not wrong". We have a dilemma.
Someone should call the director after attention has been drawn to an irregularity. If no one does, there has been an infraction of law. But anyone
may call, so if a particular player does not call, he has done nothing wrong. I think a practical interpretation of this is that once someone calls the director, the rest of the players at the table are off the hook. Still,
someone should call. The Law does not say the victim (poor choice of words, Nigel, but I'll run with it) should call. It does not say the offender should call. It says
someone should call, and leaves wide open who that someone should be.