After committing a crime from a foreign country and not serving punishment, can you return?

I have a cousin who currently lives in Barranquilla, Colombia. He was sixteen when he got his Visa/Green Card? to come to the States. Once he arrived in Florida he was enrolled in school and got into bad stuff. After being caught with marijuana he was trialed but never completed as his father sent him back to Colombia. He turned seventeen here in the States, and is now close to turning eighteen this year. I have no knowledge whether his Visa is still active or not, but I'm curious if he can never return back to the States (Florida) by law anymore; everyone in the families been saying that, but I've argued that he can with a bought plane ticket and a home here. Of course we want to get him back legally, so I'm trying to find a way.