I would expect it to act somewhat like that IF both computers were assigned a Static address that happened to be the same. But, usually if that happens, a little yellow pop up balloon would pop up in your system tray and tell you that.
And, by logging on, I assume turning her computer on?
Try checking the IP Addresses on both machines.
Start > Run > CMD > ipconfig
It should be in the form 192.168.x.x usually.