The TurboTax online editions can only be used to complete and file one tax return per account and User ID.

To start another tax return using the online editions you would need to create a new account with a new User ID - https://turbotax.intuit.com/personal-taxes/online/

No, don't use your account, if you do their information will overwrite yours and you cannot get it back.

With the Online version you need to create a new account (UserID & password) for each return.  You need to sign out of your account & create a new account for your friend's taxews.