Technically not AFKing, as he was there to respond.
There's no way to really solve this issue without implementing something that could be easily abused-- such as Vote-Kick. And I don't think that needs an explanation.
I've never really had a problem with people not contributing to a team; mostly because I plan for the worst-case scenario where everyone is useless, and use a tactic that would work just as well in solo.
What I'd recommend is you don't let then succeed in leeching (or do nothing at all and carry on). As soon as you notice someone like that, leave the game and star