We've been doing approaches down to 100' & 1/2 nm on a weather radar for some time without autopilot or altitude hold, etc. If someone is calling from a beach, we can plot their location, use the weather radar to map a point on the shoreline nearby and complete the approach.
Could be possible with IR as well, although I'm not certain how well you can get distance from IR.
With weather down to 0/0, helicopters aren't normally the best rescue platform. Hovercraft, GSAR, etc. are better.
If the helicopter can fly above the fog and stay in cell phone comms with the person in the sand, the helicopter may be able to vector the person to safety. Fly overhead, if he hears the helicopter get him to walk towards it. Risky, but it may buy time until a ground team gets in.
Matthew.