What constitutes being AFK?

I had an ARAM match where our Pantheon stayed on our backline and only ever attacked and used abilities on minions. His farm ended up being better than anyone else's, but at the 18 minute mark, he was 0/0/4, while our team had 40 kills total. We were winning anyways, so we didn't necessarily need him, but I felt that participating in only 10% of the total kills on our team during an ARAM match is tentamount to just not being there in the first place. It wasn't until I threatened to report him that he jumped in, finally died, then came back with a bunch of items and stomped. I actually didn't end up reporting him, since he did come in and kick some ass at the end.

TL;DR Even if you are doing things, is sitting way back as a melee character and not participating in teamfights (in ARAM specifically), worthy of an AFK report? Is it the same as a jungler on SR doing absolutely nothing but farming jungle until the end of the game?