There is a pill which helps with this problem. Talk to your vet. Sometimes true medical problems/deficiencies in vitamins are the cause. Some folks believe it is due to artificial flavorings/chemical compounds which mimic food smell/flavor in food which do not digest. Leaving the smell/flavorings in their poop because it won't break down completely like a natural compound. the chemical half/life is much higher than the ability of the dogs digestive system.
There are a myriad of wives' tales about tricks and this and that. The best is to just pick up the yard more often and correct the dog when you catch them doing it.
Spaniels are sensitive as it is so, folks who strap e-collars on dogs to correct them for a lot of what are just obedience issues end up ruining their dogs. If the guy doesn't want to go to the vet try a natural dog food with no artificial colors/flavors such as Costco's new fish and potato or Taste of the WIld. Costco's is about $15 cheaper a bag.
