what do you mean excatly with 'reserve an ip address'?
RRAS will assign an IP address to the VPN client either from a pool defined on ISA or from an internal DHCP server. In the latter case, RRAS grabs IP addresses from the DHCP server in blocks of 10 IP addresses. So, I don't understand your question.
If he has the same ip address each time he does not have to search for the number when he is trying to work and needs to know what the ip address is. He may be putting the number in to his host file I am not sure.
From: Sydney, AU
To enter a static IP for VPN client, you need to have your domain running in "NATIVE mode" and not "MIXED mode". Once this has been done you will be able to assign a static IP address to the logon account.