Reporting elo boosting?

How would I go about reporting a person who I know has gotten elo boosted? Amusingly funny when people try to take credit for thing they themselves did not actually accomplish. Funny when a player who always uses d as their flash key and struggles in low silver changes their flash key to f for a long winning spree and does amazingly uncharacteristically well, then once they hit gold their flash goes back to their D key and they go back to being mediocre.... Where do I file a report?