The goal is to kill the noise, so it can just be done by ear once you have the frequency locked in. Although finding that exact frequency may be difficult since high frequencies are hard to compare audibly, so a scope might help there.

But it won't work. Wave interference is highly position dependent. So unless you can inject the signal to the same emitter that's causing the problem, the phase will need to be tuned to an exact location. The wavelength of 25kHz sound is about 14mm, which is much less than the distance between a person's ears. So unless their ears are an even multiple of the wavelength, tuning for one ear will mean that the other ear still gets the signal (possibly doubled by constructive interference). And even then, that's only if they're facing tangent to the source. I suppose that means there is an orientation they could stand in where both ears are in destructive interference nodes, but they would have to be very still.

You can test this at home. Play a constant tone through two speakers and move yourself around, listening for the different nodes. You can also switch the input polarity on one speaker to switch the constructive and destructive nodes.