Broken chassis?

Can anyone help me with this question?

My stepson was driving his girlfriend's car, and "hit a bump and broke the chassis." He claims that it happened because his girlfriend is from upstate NY and either the chassis goes from cold to hot (here in Tenn) and back again, thereby weakening it, or the cold and snow up there caused rusting which made the chassis break when he hit the bump. He claims he had no control over the break, that it could have happened at any time, to anyone. Now, I don't know much about cars but I know a fishy story when I hear one. Does this make logical sense to anyone who knows more about cars than I do?

PS. My stepson knows less than I do about cars. Last year, at age 22, he was asked to bring home our second car. When he got to the car, the battery was dead (it made a clicking sound when he tried to start it up); he looked at the jumper cables in the back seat, didn't know what to do, and left the car where it was. Even I can figure out the red clip goes on the red button, etc.

Thanks for any answers,
Jeri92