A friend of mine was involved in a situation like this once. She broke out the window, then waited for the owner to return. When they did and started to protest about window threatening legal action, my friend's reply was, "Go ahead. If you go after me for breaking the window, I'll pursue animal cruelty charges against you."
I'm with the other folks who said they would have 'forgotten' to mention that the window was cracked at all. It was such a minute amount you could have very believably said you didn't see it.
Unfortunately though this didn't sound like a very caring or concerned pet owner. Short of removing the dog from his care, it doesn't sound like he was very impressed at all by your attempts to set him straight on how dangerous it is for that poor dog in a hot car.
