Brilliant thread already and what was an immediate clear cut answer to me has been muddied with a little bit of research And knowledge.There is quite a lot of barring in the inner tail feathers and the white wing patch; and what about the short but still visible sixth finger?
(not like typical red kite)
The tail fork looks rather deep for Black kite on the other hand.
So I see here where your friend was coming from when saying Hybrid .
I agree, both on bright light and wrong use of term 'eastern' by myselfSome Black Kites are redder than others and may look especially so in the bright Middle Eastern light, but I would not say that eastern Black Kites are on average any more reddish than western. On the contrary, "eastern Black Kite" means normally lineatus or lineatus intergrades, and they are not reddish, nor their shape is closer to Red Kite.
Trawling images on the web Joern, (presumably correctly ID’d?) some of the Red Kites appear to have a longer P6 than others and where that occurs it appears obviously longer than the adjoining secondary?Ken I do not understand you here - however long it is , it is always a primary concerning the anatomy?!