I have seen similar wierd things before. If you are sure your config is correct, try asking the user to connect via a different firewall (at their side), for example if it is a lap top, ask the user to connect via a smart phone, the WLAN service in the cafeteria or the local Internet café or similar. If it works I would suspect that the firewall does something that your firewall(s) does not like, such as silently dropping some tcp packets or similar that results in the client (software) not beeing able to connect. Could be a bad port in a switch or a "broken" switch as well but not as likely. Try replacing one thing at a time and you'll probably find what causes this. If it does not work, try using a dirrefent client software or similar service such as VNC etc at the client side (temporarily disabling software firewalls could be worth trying as well).
If all other clients can connect it should be something this client does differently and the explanation is probably hidden in there somewhare.
If you find what causes the problem, please write it here for future referenses.
cheers,
/e