Your friend, seaside20, griefed a grieftrap, which REQUIRES you to break at least 5 blocks in order for it to trigger a ban. ANY LESS and it does nothing, which makes the classical "1- or 2- block grief" not a valid excuse in this case.
What I want to ask is WHY he continued to keep on breaking more than just "a window?" It should have been made clear to seaside20 that he should've reconsidered his actions before proceeding to grief even more. I don't care if he's a new player-- he chose to disregard the server rules and that is his own fault, not ours.
Grieftraps do not catch false positives.