If my kid was given a ride home because she was drunk, I would thank the friend for getting her home safely and discipline my kid for underage drinking. Unless the drinking was at a school function or on school property, the incident does not concern the school. The onus for my child's behavior is on me, not them.
When my husband got home from work, I told him about this thread and we both essentially said just what you said. We discussed it from both sides -- parents of the drinking kid, parents of the driving kid. You sum it up perfectly: The onus for my child's behavior is on me, not them.
I should add, my husband has a lot of alcoholism in his family and NEVER drank at all in HS or college.