The Free/Busy lookup doesn't work internally.
The tool prompts for the UPN of a user, but treats the UPN as an email address for the purposes of an Autodiscover lookup, and our UPN suffixes are different from our email addresses.
The manual Specify Target Availaiblity Service Server FQDN appears to be ignored because the results show the tool is still looking for the autodiscover service based upon the UPN domain suffix.
Since walking a user through using a tool like this remotely will be complicated enough, could you add the source mailbox email address and use that for the Autodiscover lookup instead of having the user manually specify a FQDN of a server (which they won't understand on top of it currently not working)?