He wasn't let off - he was charged with much graver offences. The court would in all probability not impose any extra punishment if he had been charged with phone use.
As for his mother being unaware that he was using her car - well I'd be quite sceptical about that. If you own a car, you need to know who is using it. Years ago it was a case of the owner in this situation being given a choice - either be charged with permitting no insurance, or provide a statement to support a charge of TWOC being dropped on the son. Nurse, or not, I cannot see why this should not still be the norm. Had she been deprived of her car, and charged as she could have been, she would most likely take much more care about who drove its replacement.
What is the world coming to when the Police start becoming sympathetic, eh?
+1...and god forbid them using discretion