Helping a Friend with Objectives

Hi All, not sure if this is the right place to post this.

I've play a lot of coop with my friend and he's going to be away for three weeks, can I log in from his account to do their Obj so they're not left behind or will I get banned? 

Thank you in advance. 

-A newbie

If your friend trusts you with their sign-in information, the game would have no way of knowing that it is you playing and not your friend.

I do it for my kids all the time