As others have, I would very strongly emplore you to encourage them to train their dog properly, as an anti-bark collar is just going to upset and confuse it. Of course it's much harder work but it's far better for the animal psychologically in the long run, chihuauas, much as i'm personally not a fan of the breed, are very smart dogs, and will respond very well to correct training, particularly if he's still young, but even if he isn't it. Beating an animal out of a natural habit that annoys the owner through aversion training really isn't ok, and positive reward for good behavior will always, always work far more effectively than negative inforcement. If they aren't willing to properly train and care for this dog, they simply shouldn't have it, and should re-home it with someone willing to put in the time and effort to train it properly.