Hello
There is the same quotation in "carré d'as" page 99. Direct translation, same words.
For what it's worth, "Carré d'as : Guynemer-Nungesser-Madon-Dorme" was published in 1934; there is no french version of "Guynemer- the ace of aces". The domestic autorized biography was written by Henry Bordeaux
de l'Académie Française: "Le chevalier de l'air - La vie héroique de Guynemer". Guynemer's family gave him a lot of material (and beside that Mortane was just a sport journalist

).
I have never seen the picture of the falling observer. Pictures of the camera gun have been published. They were part of Guynemer's personal album but in publications are credited "SHAA" (now renamed SHD-air), the historical records of army, so I guess that's where they are now.
The air-to-air pictures are captioned and signed by Guynemer and his CO, Brocard, with the mention "certifié exact" so it seems they were intended to be used for victory homologation (I don't think Guynemer had a business idea in mind

).
However, even if the pictures show german planes in Guynemer's 12 o'clock, and sometimes at short distance (15 meters!), there is no evidence of any damage to the planes. Maybe the camera shooted its picture with the m/gun trigger, so before the target was hit, I don't know?
Anyway, as the pictures ended up in Guynemer's personal album and not in the army files, I guess the idea was considered useless for victory confirmation.
Gilles