Jay
I'm afraid, as the new claim resulted in a new Decision that was later than the Decision that he appealed, an appeal re-makes the original Decision it does not create a new one, it will that Decision that is taken forward. So the Appeal Decision will apply from when his PIP stopped to the date that the new claim started.
He can use the Tribunal result as evidence to support the MR and if necessary any new appeal that he makes.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems