Since it's July, I'm going to assume that (1) this is summer school, or (2) this happened a few months ago, and the parent is looking for retrospective advice on how s/he may have best handled the situation
Yes, the kid should/should have been disciplined. It indeed sounds intentional, and this behavior betrays a lack of consideration for other people's time---e.g.forcing his sister to miss school and forcing the parents to clean up his mess. As you mentioned, he could have easily made more people sick by intentionally exposing others to his germs.
If he did this as a teenager or young adult, he could find himself in more serious trouble. Forcing others to come in physical contact with the bodily fluids of someone with an infectious disease is technically bioterrorism :)