You don’t have to get punishment. She didn’t sneak on board; she asked and the driver made the decision and gave her a lift. She was doing the girl a favor.
Would anyone have been happier if the girl had been refused and forced to get somewhere herself while she was sick?
Doesn’t matter how she got where. It matters that she didn’t let the school know where she was. All she had to do was call the school or have a parent call and explain what was going on. Not hard to do. Lots of people do it every day.