Is this report-able? There's no way to do it.
Ever played a game where an ally deliberately wants you to die? You'll think it's a 2v2 for example (And in your head it's a 100% winnable fight) then your ally just keeps pacing behind you doing emotes and things while waiting for you to die so he can flame you some more? Usually comes from people who want to surrender. For example, a fed nasus in a losing team doesn't want to surrender, so his ally tries to leave him to die so he can surrender. It happens often between people who are flaming each other or people who are just tilted at someone who's not doing well (They end up trying to make him do worse).
Best thing about it is that there is no way to prove it and no way to report it. All I can do is report negative attitude, but it seems that they can only punish intentional feeding and certain keywords.
[zombie-nunu-tears]