If the drive is loaded before LW starts, you can set the downloads to go there. This is a bit risky though: you can't remove the drive safely while LW is running. If LW starts up without the drive, the preferences will be wrong.

Your best bet will be to talk to your Dad and work out something reasonable. You could be putting him at a small risk if the internet connection is being used improperly. Do you really want to give him reasons not to trust you?

cheers