Report them for this. Under the additional comments box, when it lets you type any extra information, specifically put that the player threw the game on purpose. It does not require multiple people in the game to report them for it to be reviewed by the system. Your report is enough to flag the game.
I don't have the client up at the moment, but I believe the first report category is the Negative Attitude/Griefing one. That is the one that I would use. Obviously, if they are not saying anything toxic, you wouldn't use the verbal abuse or hate speech categories. And if they aren't leaving the game, you wouldn't report for afk. But by intentionally letting the enemy team get stronger, or win the game, they are griefing the team and it is reportable.
This articlefrom Player Support does reference some aspects of the old client/gameplay (rip mana pots) but you can see that purposefully losing and refusing to participate are things that Riot has long-since considered reportable. We've all had that champion who sits in base dancing while the enemy is taking our inhibitors. It's actually something that Riot has addressed.
It may seem like players get away with this sort of thing more often than the toxic chat behavior, as it's not as common for someone to end up here posting that they were banned for this sort of thing. But keep in mind that it is a bit harder for the system to "prove" these behaviors. It's not quite as easy to verify that a player was intentionally throwing a game as it is to verify that they used racial slurs. And rather than punish innocent players for making mistakes, the system does try to make sure that it has a decent amount of evidence prior to handing out a punishment. But if a player is reported for this consistently, they will be punished for it.