I once bought a dreadnaught at a pawn shop for fifty bucks in the dead of winter. It was a great guitar. I brought it home and the next morning opened the case and was greeted by about 30,000 fleas. I think the owner's dog slept in the case or something. I let loose a flea bomb inside the case and it lived on the porch for three days. I eventually ditched the whole business; I had no fleas but smelled like OrthoMan every time I played.
Odor is tricky if it is from a fungus, because the inside of the guitar is porous. The first thing I would do is pull the strings, get a rag, dip it in ammonia, wring it out so it is not "wet" and run it along every surface inside I could reach. Don't let the inside get wet, though.
It's more likely that the problem is the case. If he sweated a lot and put the guitar away wet, the case would soak it up. So that's the likely culprit. Hit the case interior with Lysol spray, then when it's dried, try upholstery cleaner and a vacuum. Let it dry for a few days before putting the guitar back - it needs to be absolutely dry.