I have been diagnosed with the same thing, as when i did the tilt table test I fainted in 6 minites which was some sort of record they told me. Salt increases your blood pressure, that would be why they would suggest she eats more of it. However you don't want to eat too much of it either. I read your other post about beta blockers too-I was on beta blockers for a period of times but reacted badly to them so had to go off them. If they are going to give her a mild dose, she should be fine, and it should help her-so it's something she should try.
I think if she gets her blood pressure under control she'll feel alot better.
Hope that helped!