Have the fellow show you "exactly" where he dropped the ring.
Fortunately, rings have a tendency to settle straight down to the bottom with no lateral drift.
Take a small weight and attach to a reel. Lower the weight to the bottom and let it GENTLY settle on the bottom. (When you get down there and start probing around in the silt you'll understand why.) Make sure the reel line is securely attached to a fixed object at the surface.
Your down line will act as a reference point when you start your search.
Descend to the bottom of the reference line making sure you approach the bottom very slowly.
You can conduct your search in one or two patterns, circle in an outward expanding orbit about the down line, or start on one side of the line and do an up/cross/back pattern.
Make sure you keep your fins up and your body in a heads down position if you possibly can. A frog kick goes a long way to keep from stirring up the silt.
A ring, more than likely, is going to settle down into the silt.
If it hasn't been too long, look for tell tale signs of dimpling in the surface of the silt. Using you light you can help detect dimpling by shining the beam of your light at a very low angle along the surface of the bottom.
Good luck.
the K