Throwing a game for a fountain kill

How can I even put this into words. We have an easy win by 25 minutes and my zac and mord are taking nexus. but mord stops hitting it to fountain dive for a kill. not even getting the kill. and zac doesn't get it in time. they proceed to turn the whole game around on us 20 minutes later and we lose. because he had to have a stupid fucking fountain kill. The whole game was thrown just cause of that one moment. they snowballed and won, no matter what I did I couldn't do anything. but watch as my team throws it. how can I even report that? its not feeding? its not griefing? but that is def reportable behavior