Yes. If you set up a new log id and password, you can add your friend's family.
If you are the one working on his tree, you can go into Account, and click on Create New Family. His tree will be accessed by your logid. If your friend wants make the changes, then it's a good idea to set up a new logid. If you set it up, you can always send him a link to his family so he enter and review it. You can give him editing rights so he can make changes or not give him those rights.