All he has to do then is that he didn't deliberately assist or cheat in any way of the definition. Meh, there's a decent chance that he can appoint blame elsewhere with proof, there is already some loose proof that he has played games with the account in question, making the argument that he himself has ties to that account a little flimsy.
Of course, this is not enough to warrant that he is innocent but there is already some proof in his story, as in, he hasn't been authoring the account after the time it was purchased and given away. Also, the Finnish legislation, for example, forbids every contract an underaged person does that they don't understand, and the benefactor will have to reimburse the lost funds if this can be proved, I'm not saying that he can do that.
AFAIK, it's a FACT that he missed out on a paycheck and showcase of his life for CS already, and it can be used as leverage against Valve, not necessarily as what he has to be given compensation, but as something that needs to be weighed in as well as his age at the time of the alleged lan party etc.
There's very flimsy info flying around and Valve's tos or subscription agreement alone is never proof in court as is, they have to weigh in on how these situations have been applied to Jamppi, as I said, we'll see and get a consensus.