Another question that came in recently was that of why a person in a horn section sounded bad. In the E-mail it mentioned the player was talented (high school aged) but that the tone was very bright, loud, and out of tune.
In short there could be a whole group of things wrong and it would probably take weeks to sort it all out in lessons.
From this distance something that sounds very possible is a poor hand position. One thing I have learned over the years is how lucky I was that my high school band director was a horn player. Many music educators really don’t have the concept of hand position on the horn figured out, and even if they do students don’t always do what they say. Hand position is discussed various places in Horn Matters but this article would be a good one to start with.
My guess is the hand position is either very open, palm against the bell, or not even in the bell. This would lead to an overly bright tone and a very sharp tone as well. And if the student is used to that sound and pitch—it seems normal to them—then they can’t learn how to adjust things.
If the hand position is great than another thing that comes to mind is the general topic of tension, possibly embouchure related. You are not going to be able to say much about that to a section mate though without coming across as a know-it-all, however.
Throwing out one other random thing, you may want to check that all the slides are in the right place. This can be a problem on certain models of double horns, you can end up with valve slides on the wrong side of the horn and this leads to nothing but intonation problems.
There are probably many elements to sort out, in reality, as to why the section mate sticks out and sounds bad. At this distance it is really hard to say much more than be observant and diplomatic, you may figure it out. Good luck!