What is his connection to the net? If it is a PPPOE
try adjusting the max frame size on the PPPoe adapter to something 1400. If it is a dialup, try to enable compression on the group that he belongs on the 3K concentrator. Also, you could use the setMTU utility on the client (available on the directory where you installed the vpn client ) to say 1400 and try to connect again.