Just mute, report and move on...

Well, you can mute him so that "Its not my main role lulzies" doesn't tilt you more than their poor gameplay already did.

You report them for intentional feeding.

You move on, because you understand math and know that with 4 slots on your team where it can happen and 5 on your opponents', you will win more games because of this attitude than you will lose.

If you're not confident in reports, you can always open a ticket

Still covered by "just a bad game". Unless it was very clear running down mid inting , no chance of reports doing anything.