As others have said, this post discusses new accounts. For accounts that have already placed, the data is at Season 9 Placements Analysis, High Variance Explained
Put simply, a GM cannot push your established account more than a few hundred SR in 10 games because nothing resets between seasons. For active accounts, placements don’t affect your account any more than normal games. If he kept playing, he would have no problem getting to GM (unless he got banned for boosting).