How do you report someone you suspect of being boosted?

I could't find anything in the stickies but i have (had) someone in my friends list stuck in high bronze who a couple months ago went on a streak of shit stomping with a champion he never plays to get into silver. i thought whatever he finally made it although he eventually fell back down to bronze 2. well a couple weeks ago i hover over his name in my friends list and see what i think is bronze 5 next to his name but to my surprise when i check his profile it's actually gold 5. i couldn't believe it and sure enough there was that same champ he never plays absolutely rolling every game he played. i called him out on it and was promptly unfriended.

not gonna name specifics as to not witch-hunt but i was interested in reporting if possible, i just don't know how to.

and just some general op.gg stats to support my claim, this fellow has a bit over 1k games played this season with a 47% win rate. every champ he normally plays he's played well over a hundred times, his main over four hundred. not a single win rate above 49%. and as for the champ that got him to gold? a blistering 81% win rate sub 100 games.

long story short f this guy, i'd like to see him burn.