Depending upon the size and volume of the vent, a catalytic destructor may be the answer. Yes, they are expensive to operate but even relatively low temperature units are often quite efficient at destroying odors.

Second, I'd like to find out more about the chemistry of his emissions as the problem may be correctable with either of three possible solutions: 1) Try using an oxidizing compound in the scrubber water-- chlorine dioxide, ozone, even citric acid may be able to improve the odor problem; 2) try adding a masking substance to the scrubber water; and 3) run the EPA's screen model to find out how high he may have to increase stack height to eliminate odor levels at about 1.8 meters above the ground. This will require a knowledge of the threshold odor concentration but it may be the cheapest solution yet.

There are other potential solutions including developing a bio-bed reactor for the vents from compost, but that will require room and that's usually at a premium in most plants. But, he's going to have to characterize the stuff in the vent to find out what is making the smells.

If all else fails, try refrigerating the vents to condense the material.