To help a friend with a project, he gave me his gmail account login information. Now I don't know how to delete his email account from my Gmail list without affecting his account.
It's a bit unclear what your asking, but I'll proceed with an answer based on my interpretation.
Deleting his account from your computer will not affect his email account. That's the great advantage of IMAP based accounts.
To delete his account from your Mac, go to System Preferences > Internet Accounts > Select his Gmail account from the Sidebar > Remove (the "-" in the bottom left hand corner)