My comment of "is it in treble clef?" was largely a joke, but I thought it would be interesting to take it a bit further. Here's that interval in all clefs (or, at least, in as many clefs as necessary to exhaust all possible staff locations):
You'll actually notice that every single one of these is an augmented fourth except for one: in alto clef, the pitches are
B♯, which actually creates a doubly augmented fourth.
This is an outgrowth of the fact that, with perfect intervals, the accidentals always match, unless it's the fourth/fifth between
B. In all other cases, matching accidentals create a perfect interval, and therefore seven of the above clefs create the augmented fourth. The only clef that doesn't is the one that results in the
This just shows that the only way this is the student's error is if this example is in alto clef. If it's in any other clef, then the student is correct. QED