Can he telnet port 1723? Or these search results may help,

VPN Error 651 VPN Error 651 - Your modem (or other connecting device) has reported an error ... When trying to establish the VPN from Windows 2000, he receives Error 651. ...
     www.chicagotech.net/casestudy/vpnerror651.htm  

VPN Case Study VPN client can ping the VPN server only · VPN error 619 - Case Study · VPN Error 651 - Your modem (or other connecting device) has reported an error ...
     www.chicagotech.net/casestudy/vpncases.htm  

Bob Lin, MS-MVP, MCSE & CNE
Networking, Internet, Routing, VPN Troubleshooting on http://www.ChicagoTech.net 
How to Setup Windows, Network, VPN & Remote Access on http://www.HowToNetworking.com 
 Hi all

 I have a client with about 25 laptop users running XP and Outlook that
 connects to the Exchange server via VPN. All laptops connecting on VPN
 w/o any problem, except one.He is getting an error 651 and VPN is not
 even passing connecting www.zyzsitename.com message.

 Any suggestions on this?

 Igor