I suspect they have a browser hijacker!
download malwarebytes on your computer, put the file onto a CD (or usb memory stick for them)
Then they will have to start their computer up in safe mode (keep tapping F8 as soon as the computer is turned on, select safe mode with networking when the menu comes up, if you see the windows logo it's too late)
Once the computer has started in safe mode they should install malwarebytes from the CD you gave them, and then run a full scan with it.