Cheating is breaking the rules of the game. Making up new rules is totally different.

Tell your kid that new rules can be established at the beginning of a game if everyone agrees, or the first time an unforeseen incident occurs (what if it gets stuck in a tree?), but that making up new rules in the middle of the game is a BS tactic manipulative kids use to pull a win out of a loss.

Just tell Eli to suggest to his friends that any rule made after the start of the game should be applied the second time the situation comes up, rather than the first. Johnny Flopper is less likely to try and invoke amnesty for the next kid than he is for himself.