Bwahhahahaha
the simple answer is let them play it then call a judge over so that he can explain while the whole time trying to hold back the inevitable laughter and repeatedly use the power and tell them I hope u got 50 sprays
Friendly/Casual Play: Take it back, no problems.
Tournament Play: I will unashamedly admit that it depends entirely on how big a difference it makes. If correcting them and letting them take it back probably won't negatively effect me? Sure, I'll explain the mistake to them and allow them to take it back. If I'm losing or it could ruin me? I'm not going to tank my win to point out their mistake.
the simple answer is let them play it then call a judge over so that he can explain while the whole time trying to hold back the inevitable laughter and repeatedly use the power and tell them I hope u got 50 sprays