Matchmaking for unranked player

I'm currently Silver II, and I just had a game with an unranked mundo on my team who was in his placements. Not only did he feed enemy Fiora 0/4 in lane, which can happen on a bad day to anyone, he also made extremely basic decision and mechanical errors throughout the game. I feel confident to say that his currently skill level is bronze. So my question is how in the world did matchmaking queue him into a Silver iI game? Btw I checked his account and Mundo is one of his mains.