There is only one way to do what you want with the Blink system as it stands, and that is to get a single camera system purely for your neighbour.
Create an email account just for Blink, for your neighbour, and setup this single camera Blink system using this account. Put the new camera next to the existing porch camera, so that it shows more or less the same view, and connect the new sync module and camera to your network, and give your neighbour the credentials for that system.
Is it a great solution, absolutely not, probably too expensive. Would it be better if they had multi-user access, with different permission levels, of course. Are we likely to see that any time soon, probably not. Will this proposed solution work, for sure.