I had the other kind, inguinal (?) when I was pregnant with my first. We were out on a little ski boat, Mike was watching this HUGE house boat instead of the wake it was creating. We went way up out of the water, and came down HARD. I felt something pop then. Hurt, but not really bad. It was pretty scary though, I didn't (don't) know much about them, but had heard they could 'strangulate'. So with an extra dose of wacko pregnancy hormones, I spent the rest of my pregnancy wondering when the winces of pain were a sign of an active baby kicking or the beginning of an intestinal strangulation.
We did have a calf a couple years ago that had a very bad umbilical hernia. It came out of its skin. I think I posted about it. Found him in the field, wrapped him and his protrusion up and he was brought to the vet. He/she did just fine (much to the vet's amazment.)
I don't think it's 'work related' in the sense that you're responsible for the repair (if he gets it done.) It turned out I had two hernias to repair (with the mesh), one on each side. The doc said it's usually a congenital condition....a defect that's been there since you were born. For me it was Mike's crappy driving, and for your worker it was maybe lifting something a bit heavier than he's conditioned for.