With the iBook, I'd suspect the antenna cable being loose. It's easy to get to and no problem to check and fix if necessary. However, I can't find an article that even locates the card in a MacBook, so I can't say "this is what you do to check it."
Have you tried using some other wireless network to see if there's some incompatibility between the MacBook's settings and your home network? Have you examined ALL of her network settings and compared them individually and carefully to your Powerbook's settings?