Softened water?

[Dearth]

Yesterday I helped a friend setup a 14 gallon bio cube from Petland for salt water use we got him sand I gave him 2 rocks I wasn't using and gave him 25 gallons of my stand by salt water. Now we are just letting his tank cycle but I have a question. He can't afford a RO RO/DI unit due to cost atm and while I can make up his salt water for him he was wondering about using tap water that's been softened?

He is on well water not city water and uses water softener and his question is would it be safe to use softened water to make salt water? I know that some people use tap water with no issues but will softened water from a well do damage?

In the mean time I will make his water when I do a batch for myself any help or comments is appreciated.