If credit card transactions are happening on the wired network, for security reasons, I would be hesitant to allow a wireless network to connect to the wired network. It doesn't break PCI regulations (AFAIK), as long as you encrypt the network with WPA2, and use other security features. However, I wouldn't want anything to fall back on me if the network got hacked somehow.

I'd tell the business owner to get a separate residential internet connection at his home. These days internet access is pretty cheap and comes in various forms.