This is the thing, I think its fare to say keeping tropical fish or fresh water fish what ever you want to call it is not an exact science.
Maybe it is an exact science I am unsure now I bring it up.
But in the near 2 years I have been in this hobby I have read many threads and comments on this forum and others and there is always a variation on a theme.
What I mean is that there always seems to be someone doing something slightly different.
Be honest I have never used salt in my tanks and as far as I can tell it has not been to the detriment of my fish either.
If your friend uses salt well he obviously does not have a problem with it and has found it to work for him?
I know some people use salt in a freshwater tank for aiding in getting rid of certain nasties.
I personally would not do it proffering to give my fish obsessively clean water all the time it has served them well for 2 years but that's me.
That's one of the great things about this forum eventually you get all sides of the story so you can make an informed decision.
Good luck
Regards onebto