What is the proper procedure for reporting a suspected boosted account?

I'm not asking because I'm salty, as I didn't even meet the person in a ranked game. However after a normal match in the post-game chat one of the players said it's funny that we lost because our adc hasn't even played before. Curiosity peaked as to what he meant so I did a little research and found that his playstyle AND skill vary wildly between his ranked and normal matches meaning in normal games he tends to lose often, farms poorly, and plays a variety of champions and in ranked he turns into a faker level Katarina and Pantheon main that averages +15-25 kills a game while managing at least 200 cs by the 25 min mark and has a recently obscene ranked winrate spike doing it out of the blue. If you go back about two weeks ago his match history looks like that of a jungle main with tendencies similar to his normal matches. Before that two week change he's been stuck in Bronze for quite months and in two weeks rocketed to Silver 3.

I know saying his name here is against forum rules, so without calling him out, how would I be able to properly call attention to this account for those that might need to know?