I am not sure if it is possible, but what you need to do is change the mac address of the ps3 so it is the same as the mac address of the usb/ethernet adaptor that your lady is using for the pc.
To find out the mac address of this, on the pc, open a command prompt (when it is connected to t'internet) and type ipconfig. It will show you a 12 digit hex address, which is the mac address of the ethernet adaptor.
Take a note of this one and then (if you can) type this into the ps3 as the mac address. If you can't change the mac address on the ps3, your other options are a) connect to the pc via ethernet or b) buy a router.
If your pc has an ethernet connection, you need to find out the mac address of the ps3, and then copy this onto the ethernet card of the pc. You then need to connect to the pc using ethernet and you will be prompted to register the device. Follow this through and you will then be able to hot swap between this ethernet connection and the ps3.