Hello Doug,
It appears that one possible work around is to upgrade the firmware to increase the size of the NAT table. You may want to check if there is a firmware upgrade available for your friend's router.
It sounds like you could be experiencing a potential NAT timeout issue. I believe IAX has a 60 second refresh set by default. The router could possibly be expiring the NAT translation if the NAT timeout on the router is set to something less than 60 seconds. In which case you'd have to recompile iax2.h with '#define IAX_DEFAULT_REG_EXPIRE 60' set to something like '#define IAX_DEFAULT_REG_EXPIRE 29', or change the NAT timeout setting in the router.
You could easily do a 'tcpdump port 4569' on your friends system from the command prompt (logged in as root) to see if traffic is reaching his system on port 4569.
I hope this helps.
Thanks!