League-of-Legends – How to Access and Watch Complete Game Replays in League of Legends

League of Legends uses a mathematical system to match up players of similar skill in the “Normal” and “Ranked” game types. Source

So just like in "Ranked," "Normal" uses its own Elo for matchmaking. It's just hidden.

Most likely, though, because your colleagues are new, their opponents aren't as good as the ones you're facing now. In other words, your colleagues' opponents don't capitalize on mistakes as much as your opponents might, because you're a better player, and the people you play against scale accordingly.

That's why your colleagues can get scores like 0/17 and still win a game.

The immediate exploit I can think of is destroying Teemo's mushrooms. Since they last for 10 minutes, a spectator could direct a player with an oracle's where to go to clear them.

A spectator could also help keep track of buff respawns, though this is typically handled by the jungler.

Though not directly useful, a spectator can watch a player's play-style and make best guesses about future moves. Any pattern they see could potentially be used against that player.