At low doses, nitro doesn't exert significant effects on the arteries. It'll be at higher doses, like if the person is on a continuous nitro gtt that you will see those effects.
Even if, hypothetically, the single spray of nitro was enough to dilate the aorta and if, hypothetically, this relieved the left ventricular outflow obstruction for the three minute half life of the nitro, this wouldn't necessarily be a bad thing for the patient, since left ventricular outflow obstruction is their whole problem in the first place.
I think it's good to question any order that seems unsafe; it's what we're supposed to do. But, I don't see this as a black and white, right or wrong. Doctors disagree all the time, and most of the time, both points of view are completely valid. That the one doctor deferred to the other doesn't prove which one was right or wrong, in my opinion.
Again, good job advocating for your patient, but I'd also like to caution you to be very careful playing doctors against one another.