Tell your friend to be sure to check his email account associate with the account that was banned. If it was suspended due to a rule violation, there will be an email there telling him what he did, and why the account was banned. He needs to be sure to check in the spam and junk folders as well, as sometimes things can end up there by mistake.
Or, like Kei143 mentioned, if there was some reason that the account was locked for his protection, then it may not even be actually banned, and Support will just be waiting for him to contact them with proof that he is the account owner.
Whether his account was actually banned, or simply locked for his protection, I strongly encourage him to submit a ticket with Player Support so they can clear the matter up. Please be aware that Riot handles tickets on first-come, first-serve basis. It may take them a day or so to address his, depending on how many they already being worked on, but they WILL get to his, and making additional ticket.
Best of luck for you friend.