Try using a Dynamic DNS service for your mobile users.
Threre's a couple of free DDNS services out there that let you have a host name you can use to for this. Basicly the way it works is - you create a host name like "corpuser1.no-ip.com" and everytime the IP address for "corpuser1.no-ip.com" changes it will update the DDNS server so you can alway reach that mobile user regardless of IP change. Eventually in order for this work your mobile user needs to install a small program that will constantly look for any IP address change. Once it finds an ip address change it will update the server to reflect the new ip address. I think it takes like a three minutes or so to get the new changes.
Since you're only allowing your mobile user to relay by ip address or host name. All you need to do is add that host name to your list. This way all you need to do is have the host name there regardless of any ip address change. It makes like easier doesn't it?
Here's the link: NO-IP DDNS
Go for the free setup and register. Once you're there just create the host name, and make sure you download the program onto you're bosse's laptop. Here's a tip: make sure you set the program to automaticly start on startup, this way the IP address is always updated when your Boss dials up.