You really should rephrase the question.
If he intentionally let his friend play on his account: Definitely not - first, that in itself is also against the rules. Second, you're responsible for everything done on your account (i.e. even if you get hacked and the person gets you banned, you're still technically responsible. Riot may be understanding, but just saying).
If he didn't intentionally let his friend play (e.g. he left himself logged in, the friend started a game when he wasn't around) then there's still the second point that he's responsible for what happened on his account. That said, Riot may be more understanding in this case (i.e. apply the hacker-scenario understanding) and might be willing to reduce the punishment with a reasonable discussion (including a lesson on making sure no one else plays on your account).