So, the guy is a friend of an IRL friend of mine. We used to play some CS:GO together from time to time, along with other friends.
He used to be GN2 for a pretty long period of time, only to see he ranked all the way up to LEM over the course of 3 weeks.
At first I thought he used a rank boosting service, but he told me that he played with some higher ranked friends, winning most of the matches.
Fast forward a month I checked his profile and saw he had a VAC ban on record | 0 day(s) since last ban. CS:GO is the only VAC protected game in his library so there was no doubt the ban was for CS:GO.
I asked him why did he get banned and he told me he let a friend of his to play on his account, friend who later used cheats and got him banned. After I took a look at his profile I saw that wrote in his info box that the ban was for griefing (thing that is not possible due to the fact that griefing bans are game bans, not VAC bans).

TL;DR: He ranked up from GN2 to LEM in 3 weeks. Got VAC banned. Has 2 different excuses.