1st - Login to the clients physical PC or remote login while using their Login details.
2nd - .
3rd - Select add, type in the person you want to use/edit the calendar. Then select ok and click what type of permissions are required.
4th - You have to accept through the email of the person wanting access to someone else's email.
LAST
DONT FORGET THIS IS THE ONLY WAY TO REMOVE THESE PERMISSIONS.