Easy way to tell if someone shares their account
http://i.imgur.com/QegmNEo.png
You guys should check IP addresses and see if their flash key differs based on IP, then put account under investigation if their performance/champion preference varies greatly between each IP.
There is pretty much zero explanation other than elo boosting for people like this guy either feeding 10+ kills or solo carrying based on whichever key he puts flash on
btw ik in the picture i got the D and F backwards so plz no point out