As has been pointed out, removing part of the roof is futile, all you will see is the top header of the wall.
The owner is going oto have to decide whether or not they want a proper fix done here, as another trap out will still leave the old combs in the wall.
I'd insist on access from inside, as much as necessary to completly clean out that cavity, then fill it with expanding foam insulation.......or walk away from the job.
Anything less will lead to repeated episodes.....same story different year