This occurred when using the AlwaysInstallElevated policy. If they remove the AlwaysInstallElevated policy, the installation completes successfully.

Windows Installer always errors when trying to register a file on a mapped drive when the AlwaysInstallElevated policy is set. Using UNC paths instead does not produce the error.

Resolution: 3 possible ways to workaround issue
1. As you need to be an administrator to install WPS, it is not necessary to have the AlwaysInstallElevated policy set. They could add the user to a different GPO that does not include setting the AlwaysInstallElevated policy

2. Currently, they already have a script that does some preliminary things then executes the WPS client install. They can modify their script to unset the AlwaysInstallElevated policy

3. Use UNC paths instead of mapped drives as this appears to be an issue with Windows Installer and registering a file on a mapped drive with AlwaysInstallElevated set.