This is not as bad as you might think. If your wifes laptop connects through a router then the router uses NAT - Network Address Translation. This means only requests from the laptop are allowed to reply back to the laptop. An attempt to access the laptop from the internet will be blocked by the router as it will not have an originating address to direct it to.
That said I would continue to use Windows Firewall if only to stop pop up messages about No Firewall. The next Windows update will probably address the issue.