He can try that but if it is closing to within 0.004" to either side of zero there is probably either an obstruction inside of it, a gear going bad, or some other mechanical issue I would be willing to bet. Dropping them is usually the culprit when one of my guys come to me with them like this. Of course, no one knows how it happened. I'm usually a fan of just sending it out for repair, inspection and re-certification so that I have a fairly high degree of trust in the equipment. But I make my money off of equipment like this so I may be over the top for the intended use he has for these. Still, with calipers being +/-0.002" accuracy to begin with he could be 0.006" off on a measurement in either direction at any given time, and that is a lot, Could be more if incorrect force is applied while measuring. It is also noteworthy that just because it does return to zero that does not mean that they are accurate along the entire measuring distance, which is more common that most think. They really need to be given a good cleaning and inspection just to be sure.
If he is using them for reloading then they need to read as accurately as possible.