My tenant e-mailed me that the kitchen sink drain, non-disposal side,
smells.

I went there and confirmed it.

He had previously tried to find the source of the odor by removing the
pipes down to the P-Trap. He had run various cleaning liquids down the
drain which worked for a while but then the odor came back.

The odor is not from the connecting pipe to the disposal. It is from
before the P-Trap. It is not in the pipe from the drain to the P-Trap.

He said that maybe there's something growing between the drain opening
in the sink and the strainer body, or in the strainer body.

There is also a drain hose from a reverse-osmosis filter but I sniffed
that and there was no odor.

Any thoughts?

Shine a light down that drain and see if you can see a water surface in the P-trap. If not, then it's very possible that sewer gas is coming up through that empty P-trap and causing it to smell bad.

Normally, when a sink drains, there's a vent pipe that allows air in behind the draining water, thereby preventing the suction behind the draining water from sucking the water out of the p-trap. However, in the case where the sink is in an island, it's simply impractical to have a pipe coming vertically out of the island into the ceiling above. In those cases, builders will install something called an "AAV" or "Air Admittance Valve". The AAV will always be located downstream of the P-trap.

An AAV's is nothing more than a spring loaded check valve. If the suction created behind the draining water is sufficient to suck the water out of the P-trap, the AAV opens and allows air in behind that draining water, thereby preventing the water from being sucked out of the P-trap. It's that water remaining in the P-trap that acts as a physical barrier to prevent smelly sewer gas from coming up into your house through the drains in your house.

I'd fill that sink full of water, pull the drain plug and after the sink has drained, shine a light down the drain and look for water remaining in the P-trap. If you find none, then you might have an AAV valve that's stuck closed.