If you're friend got banned, every win you have that you're with him will be gone.
For example, if you have total 60 games with your friend and you won 35 games. So that's 35 Wins - 25 Loses.
If your friend will be banned, 35 Wins will be deducted to your total wins and you'll be derank if it's too much. I don't know if the loses will be gone too.