I wish to check, If i had already a google adword account, and my fren need my help to set this adword account, can i login to his google account? WIll i get ban for it ?
Create an adwords My Client Center account where you can manage more than one adwords account. To be on the safe side, check if your friend faced a google ban anytime. If he was banned before then I advise you do not touch that account.
Creating a My Client Center account is the best long term solution if you plan on doing this regularly in the future for multiple accounts. If you just want to do it a few times you can login under your friends account and nothing bad will happen. I have done this several times.