That really depends on the architecture of the plane's computer network. This vulnerability could have been avoided by isolating the in-flight entertainment system from the flight control system. In my opinion, the person(s) who thought it would be a good idea to put both systems together is/are way more guilty than the guy who exploited this glaring flaw.

As pichlo mentioned, the article is lacking details so it's not clear how much knowledge or experience this guy has with planes. This means that there is the possibility that he knew exactly what he was doing and therefore never put the safety of anyone at risk. He appears to be collaborating with the F.B.I and he hasn't yet been charged with any crime. This is unusal. His discovery may well prevent future terrorist attacks.