I cannot imagine how you can shoot any double with the thumb that high up the grip ? I just tried to do it on mine and it is just plain un-natural unless you have enormous hands.

But I supose it could be done, It would hurt like hell though I would have thought having been smacked by a trigger guard on the second finger a time or two.
I do not wish to be rude but if that is his technique for shooting a DG double then he really needs to consider his options before putting himself in harms way.
If his departure is imminent then I would rturn the rifle (because of the excess headspace) and borrow a large caliber bolt gun for the trip.

Regards