i know it should not matter, but unions exist to ensure such unfair dismissals do not occur. as do laws come to think of it, so it sucks that something like this is happening.
I suppose it would depend on the type of game that was played. a game of touch would be reasonable but a game involving fire, weapons or one that can cause bodily harm would not.
without having to call in family favours that is all i can say for now unfortunately. Although I was not aware that one warning can result in instant dismissal - which has to be reserved for the severest breaches of contract/ethics.