I agree with Zarg, actually.

It's like when I was 14 or 15 and my parents went to Ohio to take care of some stuff for my grandmother - they decided I could just stay home this time. They took their F350 & left the keys to their Explorer sitting on the kitchen table. Sure, I had driven a few times - with my mom and then when I was little and would sit in my dad's lap. Anywho. So, I took the car, picked up my friend, went for a joyride. Got in a shit ton of trouble when we got caught - dunno what happened to my friend. She got put in Juvy or something 'cuz she already had a record. Was it theft? Absolutely. They never gave me permission to take the car.

So no, the mother of the son -isn't- at fault because she never gave permission - verbal or otherwise - to actually drive the car. However, since she'd given the kids the keys, she should've kept closer watch on them.

It's also kind of like when these kids in a Civic exactly like what my brother was driving was speeding on an icy freeway - their rear end hit the front end of my brother's car, crushing his foot into like 70 pieces (and later causing him to have 3 surgeries on it). My brother had only been going like 15 mph, but 2 kids in the other car died. Parents tried suing my brother over their deaths. Kids shouldn't have been speeding. On an icy road, no less.