To recieve digital invoices, doesn't matter what computer he has, or what type of OS, it all comes down to how much he wants to spend on the program that keeps that information. A free program would be fine, but only for a short amount of time, he's gonna want some thing far more advanced. For instance the ability to network it and have VPN availablity. He's gonna have to pay for the service, and the tech support for which ever program he uses.
If it's paper invoices, then same as before, just have to type each invoice in, which is 90 percent of all invoice recieveing buisinesses out there. You have to be more specific when asking a question, or it might not recieve many replies.