What is standard kitten behavior?

My cousin recently got a kitten. He hasn’t been to the vet yet, or been fixed – and quite frankly I don’t think he will go. She doesn’t understand how important things like that are. She also thinks that he’s “Wild” and “Crazy” when he want’s to play by jumping and using his claws. She doesn’t even let him win when they play, and she is to rough ( her excuse is that he’s being bad). But the thing is, when they play that’s helping them grow. My Dad says that if you don’t let them win, when they grow up they are mean. How do I make her understand that when he climbs up your leg, that’s just things kittens do, until they learn not to? How do I let her know what standard kitten behavior is?