I believe what your friend fails to realize is that the 613 laws of the OT were given to the Israelite people. They had a theocracy{God rule} which was incorprated in everything they did. They were told what type of foods to eat, or not eat, what kind of clothes to wear, or not wear, how to deal with diseases, childbirth, sin, death, cleanliness, cooking. Almost every aspect of their life was regulated by a law.
The capital punishment laws were probably not used as often as the law required. How many people would actually want to be the first one to throw a stone at a guilty person? The accusor was required by law to do this. As far as a child being put to death for mouthing off to his parents; this is not what the passage is teaching. This is a child who is continually doing things against Gods laws. He may harm others because of his abusive temper and actions. This isn't just a kid who says they hate their Mom and Dad occasionally. As far as the man who was stoned for picking up sticks on the sabbath is concerned, there are times when God required the extreme measure of the law to be used to show His people how ugly sin is and how they are unable to keep even the smallest dot or tittle of it.
There were probably many people who didn't keep the sabbath perfectly and they weren't killed.
Tell your frien we would all be dead if we had to keep the law perfectly. The wages of sin is death, after all.