Depends on how long has the grout been done? If it's just been finished and now he's noticing the sunken joints, I'd directly add more grout over the top of it. But.....
A problem with grouting a thin layer over cured, dried grout is that the moisture will be sucked out of the new layer really fast and will promote a weak bond. Plus it only has the face of the old grout to bond to. When dealing with cured, dried grout, scratch out at least 2/3rds of it before re-grouting. That way it dries slower, and bonds to the sides of the tile. Removing reasonably fresh grout with a grout removal tool is a dream compared to old, hard grout.
Mix the new grout to a more proper, thicker consistancy and you will notice much less shrinkage.