If his adc spent half the laning phase flaming that poor guy, and the other half of the game, (after he had enough and afked) asking both his own and the enemy team to "report for afk"...
...then what will happen is this: No, I will not report the AFK. Leaverbuster is taking care of him anyway.
What I will do, is **report the guy who harassed him to the point where that poor player left the game**. Report reason will be both Unsportsmanlike Behavior and Verbal Abuse. Subtext will read: _"Extreme verbal toxicity and report calling on multiple occasions against his support."_
And yes, this is a valid reason. Him going AFK does not excuse or justify your behavior.
I have no sympathy for people going afk. But: I have even LESS sympathy (that is to say, none at all) for Flamers.
Edit: Oh, and a fun sidenote: If the offender decides to write a line like this:
_"/all XYZ afk gg i wait in base push mid end fast please"_
...he will get reported for AFK, on top of U.B. and Verbal Abuse. Because I can, and irony is great.