I am no wiser now than before.
There is no set minimum for the wall between window and roof.
Often the eaves boxing can come down to the window head in 1970's houses.
The issue is whether or not the structure of window head + wall + wall plate can securely transmit the roof load to the ground.
You're right I cannot say more without inspecting but the advice you've been given makes no sense.
I don't wish to cast aspersions on the person involved, but can I respectfully suggest that comments about structural remedies, that don't rely on either:
(i) opening up to determine the cause or
(ii) having an engineer inspect,
should be taken with a large pinch of salt.
This is the more properly the domain of a structural engineer and when you look at some of the disasters posted about on AAM you'll see its money well spent.
ONQ.
[broken link removed]
All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.