"Configure IPv4" keeps changing. By itself. Why?

I need to use a static IP on someone's computer, but it keeps changing itself back to a self-assigned IP. Any ideas why or how I can stop this from happening?

I need a static IP so that I can set port forwarding up, so that I can use Remote Management to work on their computer, so, on their computer I get them to go to System Preferences >> Network >> Airport >> Advanced >> TCP/IP >> change "Configure IPv4" from "using DHCP" to "using DHCP with manual address" and set my IP address to the correct value... >> OK >> Apply. This works perfectly, I can connect and work on their computer just fine.

A day later (without anyone doing ANYTHING in system preferences) I try to connect again and can't, because their computer has changed it's "Configure IPv4" value back to "using DHCP" and thus assigned itself a different IP address, and thus the port forward doesn't work. (They could possibly have restarted their computer in this time - not sure) So I get them to change it back to "using DHCP with manual address" again and reenter the IP address I want again and voila! We're in business again... for a few hours. Then it has reverted again, I have to get them to set it to use a manual address again. And again. And again.

We are both using Macbook Pros with Snow Leopard. I am using a VNC program to connect to their computer such that I can install software, setup mail etc for them, as they are largely computer illiterate.