She is wrong because the softener is not adding "salt" to the softened water, it adds sodium while the chloride goes to drain with the use of either product. Plus salt doesn't get out of the softener except to drain so it can't corrode your pipes or fixtures.
You can also control the amount of 'salt' used per regeneration; all softeners allow that if you know how to do it. You control the volume of water going into the salt tank. You get 3 lbs of salt dissolved per gallon of refill water. With potassium you may need to use up to 30% more than if using salt. Resin is made in the sodium form, not potassium form (there is none), although it will regenerate resins, it is not as efficient.
The limited availability of potassium has been going on for a number of years, for various reasons; one is due to China buying up all they can get.
The discharge water should be going into the septic/sewer system and not simply onto or into the ground.