I'm not familiar with any Fax to Email features offered by HP printer models. Normally the HP models I deal with do normal scaning to email/network folder and basic faxing, not a hybrid of the two. You might be out of luck on this one unless someone else can post otherwise. Could be worth a call to HP tech support to see if they have any official info on this subject.
Hope is not lost though, we get by with something else. If this printer will have internet access then you can have your customer ditch the traditional phone line and setup an online faxing service instead. With online faxing all you need is access to an email account and then the online fax vendor handles the physical sending and recieving on your behalf.
All your customer would have to do is send a scan to email message from the m475 to a special email address and the online fax vendor will take care of the rest. Recieving works the same way. When someone wants to fax to your customer they send it to the online fax vendor who then forwards your customer an email with attachments. Some email management rules can then be setup to recieve and forward emails from that address to individuals or a shared area where all can review.
Online fax vendors tend to be price competitive with normal physical fax providers. Most of them can port over the same faxing number so there is no shock to your customer's customers. For more info check out this link: