Authenticating from command-line when "su user ..."

You can't do it with 'su'. Sudo will accept a password from stdin with the -S option. Regular users generally have no business running commands as another user so no one's written this massive security hole into the code, but you can always add them to the sudoers file if that's acceptabe in your environment and adjust sudoers so they can run a few commands without authentication. See the sudoers manual for details.

You can also explore the set-uid bit and, if it's a maintenance script, put it in the users crontab.