I just had an elo booster in my lobby who said he was elo boosting his friends account. His elo change was +450 and he went from 900 to 1700 in a week.
http://support.leagueoflegends.com is going to be your best bet on this one. Our team will look into it. I've removed the LoLking link to protect privacy
How to report elo booster?
LolKing link removed -Damiya