This is a common problem that can result from many different causes. You don't say what type of virtual private network your customer is using or in what configuration, so it's difficult to give specific advice, but here are some general observations.
Outlook will freeze when part of a remote procedure call (RPC) packet is missing. Thus, one likely cause of this problem is a misconfiguration of the maximum transmission unit (MTU) on the interface. Try using ping (ping -f -l [mtu-size] [server]) to determine what the proper size for the MTU of the VPN interface should be, and then set the MTU of both the client and server to that value.
Another fix that can help is to change the registry so that Outlook uses TCP/IP only. This change should be made on the client.
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