does it really smell like fish?
or just a foul odor?
check the garbage disposal it it has one.
we've ran into units where we had to tear out parts of the wall or cabinet bottoms to remove dead critters.
(usually chipmunks or mice)
does he use rat/mouse poison?
Another source of foul odor we encounter from time to time..
If the unit has sat empty long enough, the water in the drain traps will evaporate and allow sewer gas to enter.
Just a matter of pouring water in the drain to fix that.
and lastly, one of our prop mngr accounts had to evict some undesirable types.
as a "going away present", the dirtbags stuffed two big catfish in the washer and dryer.
They also went out to the dumpster and brought trash IN to dump all over the unit
..L.T.A.