The young lady from State College PA who called about her “ex-boyfriend’s” (or should be) Mustang got (unusually) correct advice from you two. Yes, you described the operation of a differential, and how it would leave two divots on an evenly gripping surface, but one divot on uneven slickness. It probably did this randomly, but he didn’t observe well enough.
However, the more important issue is why she is still with a doofus like this.
First of all, he had no right to accuse her of damaging his car. If he understood the operation of the differential he would know it was not damaged. As an engineering student he should have known. More so, he was not being very “boyfriend-ish” in accusing her.
Secondly, his immature habit of “peeling out” of a gravel driveway is causing more damage to his car, and any other in gravel receiving distance, than she could have.
She mentioned that he is a civil engineering student, and her family has mechanical engineers who correctly told her she didn’t damage the car. This brings to mind the old statement (joke?): “Mechanical engineers build weapons… civil engineers build targets.”