I don't believe it is a UDP key, although it does translate to an IP Address. MC does a HTTP lookup to this address when the Access Key is used:
http://wr.jriver.com/libraryserver/lookup/[Access Key] to get the IP Address list to try. Certainly using a URL on a LAN allows MC to connect correctly. Also, if he can edit tags, it would seem that he is connected correctly.
Regardless, you could just try using the Access Key method. Not doing so is just being stubborn. It doesn't get a DNS lookup involved, so it may make a difference.
I'm pretty sure it has something to do with MacIntosh gear and their driver...
It may well be. What is his Macintosh preamp with integrated DAC, and driver version, specifically? Maybe someone else has had a similar issue with the same gear.
But if it is the Macintosh gear, it is very strange that local versions of the same files play correctly. I don't think anyone here or from JRiver can help, unless you can identify what is different in playing to the ASIO driver when connected to your Library.
Did you get him to try playing to just the local speakers in his PC, as I suggested a couple of days ago? PC speakers would be the simplest audio device on his PC, so should work. If they don't work, you can probably eliminate the Macintosh gear as a cause.
I'm sure plenty of people have MC Clients on a LAN using ASIO drivers, and that works. His drivers and hardware work locally, just not when files are served over the internet. So I suspect it is something in the network path.
Maybe the MC logs say something about why playback fails?
Maybe something in the network path is blocking some packet types and causing the failure.
Maybe there is something particular about your Domain, your ISPs service or their routing and filtering, the DNS server you use.
Maybe using Wireshark on his PC could identify an issue?
Maybe...