You shouldn't be registering it for them, because part of the registration is the declaration that the use is for Home and non-commercial use (and both must apply) and you can't make that declaration for them
The avast for home comes with a demo key, ready to use on 60 days unrestricted trial no need to register immediately, but they must be the ones to register. If you register using your email then you will be sent a copy of the last key you were sent (if it is still in date) to that email address.
The registration is relatively straight forward and you could probably give your friend some help with that, I would assume that they do have an email account and address ?