If this user is configured locally on the concentrator, then go under Config - System - Address Management - Assignment and check the "Use address from authentication server". Leave the "Use Address Pools" box checked.
Then go under Config - User Mgmt - Users, modify this user and on teh General tab assign them an IP address and subnet mask directly.
The fact you have checked the "Use address from authentication server" box, in this case the 3000 itself is actually the authentication server, and so it assigns that address. All the other users who don't have IP addresses defined default over to using the address pool as normal.
Just make sure you remove that persons address from any address pools you have defined.