Your customer's water softener is not producing soft water. The brine tank may be filling with water.
A water softener is used to protect clothes from iron and manganese staining, plumbing fixtures and appliances from mineral deposit damage, and reduce household chemical usage. Your customer has spotting on their glassware, cloudiness on shower stalls, and dingy clothes. Also, they may be getting iron and manganese staining on their clothes, appliances, and bathroom fixtures.
The most common problem is the brine used to regenerate the softener is not getting into the unit. The first place to look for a problem is in the "brine injector." Just like the water aspirator you used in elementary chemistry class to make a vacuum, the water softener valve uses water flow through venturi across an orifice to pull brine from the brine tank into the water softener. If anything plugs the orifice, the brine will not get into the water softener.