If it is true that she has T&G 1x6 decking as her entire roof, that would be very very rare for a residential structure that is not cathedral.
It would also be awesome because all roofing nails will take hold in a solid piece of wood.
Around here, all the old residential structures
Have Tounge and groove 1x6 or 1x4 all around the edges of the roof.(exposed soffit areas)
With regular 1x6 decking inside the attic.
This photo only shows 1x6 decking.
I cant see if it is T&G 1x6.
I would need to see a close up.
If it is not all T&G decking( much more likely)
You will have an airspace every 5 1/2 inches
Which means their nail guns are going to
Be hitting air spaces all over the place.
A great way to counter this issue is to have Ice and water shield self adhered underlayment installed over the entire roof.
A small upcharge over other underlayments
That helps seal around the roofing nails that will hit into these airspaces.
Your homeowners insurance is required to lower the cost of your policy when you install this underlayment.