Check his IP when connected and see if its overlapped with your guest. If they are not then ping the resource he is looking for using the host name and see if it returns an overlapping IP address. If they know the resource IP then just compare. If they dont know the the host name or IP because it's on the backend and they dont have access to that data then thier IT department should know. Honestly it would be easier to tell the sales rep "Here is the guest network subnet, and your guys if they overlap." Otherwise set up a different SSID, build the gateway on the MX, enable DHCP, and assign it a different VLAN ID. If you can build it so that the traffic is completely in no way shape or form on the guest subnet that will eliminate the issue very quickly and require no contact with the various vendor IT administrators. Which ever way you go I would start by eliminating that variable up front.
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