Very good suggestions but I want to drive home the fact there is no solution whatsoever unless he removes the carpet from the vehicle and addresses the woven padding that is glued to the carpet on the backside. Because I am an asthmatic since birth I go to the extreme of peeling the padding off and soaking it in vinegar and borax solution with water combine all three in equal amounts. Or bleach solution mixed sparingly so you don't overdue it. Another product that is hard to find in my area is moldex.
Then shampoo both sides of the carpet with Vinegar / Borax / Hot water, and rinse. Here is the secret: Allow it to dry in the sun and shampoo again with warm water and a commericial carpet extractor fluid if available or wool lite. Do not add much to the water. Over doing is way worse as the left over detergents will attract more grime and odors.
Reglue the dried backing to the carpet and let the glue offgas until you cannot smell the fumes.
Clean the floor boards with a bleach solution and allow to dry without rinsing. You can also use microban non scented.
As a really nice finishing touch, crack a window 1/4" and and place and ozone machine in the interior blast it for 24 hrs. and the final 2 hrs you should run the max AC setting with a battery charger connected to get any mold spores out of the A/C coil. **Do not** breath the funny ozone smell. Let it offgas with plenty of ventilation. In one week the interior will smell normal again and the mold smell will never return.