Several viruses can infect garlic, and because they are hard to tell apart, they are called mosaic virus or dwarf yellows, among other things. Virus infection would be my first guess.

That almost looks like herbicide damage, perhaps from nearby lawn treatments? But it could well be a virus, too. Hard to say at a distance.

Does the garlic recover?
Does this happen about the same time every year?
Does it happen wherever garlic is planted or only in specific places?
Is garlic the only thing affected?
Does he use fresh, clean seed stocks every year and have it happen anyway?

Just a few questions to help isolate the problem.
I have not seen this sort of problem in my own garden; so I am more or less guessing.

This looks like Aster Yellows, which devastated several growers in Minnesota a couple years ago. Really no cure for it, don't recall if it can overwinter in the soil. I know it is spread by leaf hoppers and a number of other insect vectors.

Do a search here, there was a fairly extensive discussion of the problem.

Steve