I'd go to the school and have a meeting with the other kids parents. Personally, I'd hold the kids 50/50 responsible. The owner for leaving his books, of course. But when the son picked up his friends forgotten backpack, he took responsibility for it. He then left it somewhere that it was taken and the contents destroyed. That is how I would view it if my children were involved.
ETA: if the other parents couldn't work it out, I WOULD pay for it. Again, because when my child took the backpack, they assumed responsibility, the same if they had borrowed an item and it was stolen. The backpack should have been turned in to a teacher or given back to the friend who forgot it. Instead, it was left outside where it was stolen.