Ah! You are sharing files, and not sharing your whole account - good good. So here's what you do: in the Web app, bring up the sharing menu by clicking on the sharing button in the bottom menu for the folder you have shared. A menu should appear between the left menu and the main pane with your friend's email address listed. I think the checkbox there is checked - unchecked it. Now your friend, when he signs in with his account, should NOT be able to write, only read. J