Does your wife's PB AirPort card have the very latest firmware (installed through an AirPort upgrade)? Also, what sort of WPA are you using? There are a number of flavors of WPA, and some of them are not necessarily supported by older AirPort cards, even with the latest and greatest firmware.
With the really small amount of information you've provided, that's the best idea I can come up with. More details are imperative; your specific wireless router (brand and model), which MBP you have, details about your WPA settings, and so on.