The crux of what you are aiming to do is exactly what Gravity Flow’s user input step was designed for. You can configure which fields of the form are editable or displayed for any given step in its’ workflow.
So in your case, Person A fills in the form (and provides the email address or WP user that is Person B). When person B accesses the link (to an inbox page specific to their entry), they would only be able to modify the fields you have defined as editable in the step settings.
With the latest release of Gravity Flow, you can also define editable fields as a part of an approval step. Perhaps Person B revert/rejecting the entry would be appropriate for Person A to make changes to their initial submission values?