Changing a user's password
How do you give a user a new password? Well, in the "User & Groups" control panel, you select reset password for them. What if they've lost or changed their email address? In the same place, set the new one. What if "that doesn't work"? You can assign them a new password. This is actually easy to do but I always forget how and for some reason it is difficult to find in the documentation.
In the ZMI, go to:
*<root of plone site>*/acl_users/Users/acl_users
The acl_users folder is a bit of maze, so make sure you go to its Contents tab and tunnel down from there into Users. This should take you to the list of individual users. Click on the name of the one you need to fix. You'll have a form there that will allow you to directly enter a new password.
Make sure it's insulting.