A mate of mine has had a whole house water softener fitted in a cupboard
next to the sink and all works fine as far as soft water is concerned.

Anyway there is a salt tank overflow pipe in case the tank continues to
fill, that allows the excess water to drain away. The plumber has routed the
overflow pipe (16mm clear plastic ?) to the cupboard next door and through a
hole in the "bung" (also containing the 40mm sink/dishwasher waste pipe) in
a 4inch waste pipe in the concrete floor. There is a trap underground.

This appears a nice neat solution, but when the sink/dishwasher empties
smelly air from the drain (above the trap) is forced out of the water
softener overflow pipe and makes the cupboard stink. He has clamped off the
overflow pipe and smell has stopped.

He can't route the pipe outside as there is no outside wall convenient,
basically a conservatory is in the way to the nearest outside wall.

Any thoughts ? Are there one way valves that will let water out but not let
air in ?

Can't really use a trap as there is no flow normally and would probably dry
out and let gas through again.