A donor/owner of a PCP has lost her password. How can I help her?
The precise steps to reset a user's password will depend on which CMS platform you are using (i.e. Drupal, WordPress, or Joomla). The CMS — not CiviCRM — manages user logins and passwords.
Nevertheless, the first step is usually to find the user's contact record in CiviCRM and click on the link to the user's associated CMS record as shown below.
Once you've reached the page for the CMS record, resetting the password will hopefully be intuitive. If it's not, then I'd suggest searching the web for something like "Reset Drupal user's password" (or similar, for other CMS platforms).
If you don't see a "User ID", as is shown in the screenshot above, then it can be a bit tricky. Most likely there will a different (duplicate) contact record in CiviCRM which will have an associated CMS record.