I don't know why you used the off-line registration, unless the system doesn't have internet connection.
It does sound more like you got a copy of the old registration otherwise it would have had 365 days remaining. But surely the off-line registration would be to the clients email address, if that hadn't previously been used then the resend wouldn't be an old registration.
I don't know how you go about installing avast free on a clients machine, as part of the registration is essentially the user confirming they are eligible to use the free Home and non-commercial use and you can't really make that declaration for them. You could install it for them and they have 30 days unrestricted use (aside from the register or upgrade pop-ups) to register it.