he didn't get why his friend would do that when again he himself never would because he knew it wasn't right.....
and yes, this simple incident at lunch time ruined the rest of his whole day== he wasn't able to get over it.
I wouldn't be surprised if the other kid did this on purpose, knowing the kind of reaction your DS would have.
People with autism have a hard time understanding when rules can be (or are) bent or broken. The book Parenting Your Asperger Child has a chapter on the "rule boy" type. It is worth cultivating flexibility deliberately, because without it these kinds of incidents keep occurring. This can be very stigmatizing as the child gets toward adolescence, and different rules apply to different social circumstances.