I can understand your concerns, however, I do not believe them to be incorrect. I agree that the dissonance is fairly pronounced, particularly against the Solo Horn note, but it is part of a recognisable chord progression, and I think the clash, which resolves on the 3rd quaver of the bar, is intentional.
(I don't want to appear to show off here, nor do I want to appear a bore, however the chord is essentially a first-inversion G7, (band pitch) with the 1st Trom. playing the 7th; added to which the 5th is flattened (Flugel) and the Solo Horn plays an accented passing note, which resolves onto the root. - it may well still sound 'wrong', but I think it is technically 'right', just a bit extreme!)
Regards,
G.