Should I Do Anything Extra to Protect IPhone After Going to Beach?
I think your friend is confusing sea shanties with reality.  What old mariners call "salt air" is really just the smell of decaying seaweed and dead fish.

Salt, being a solid crystal, can't evaporate, so if there's any "salt in the air" it would be in the form of salt rocks flying around.  I imagine that's probably a lot less likely than "sand in the air", since there's typically not a lot of unsequestered salt at the beach.  Also, I put salt crystals in glass all the time (e.g., measuring cups), and have yet to see the salt scratch the glass at all, so I think the chances of this hypothetical "salt in the air" causing damage to an iPhone are practically nonexistent.

I would worry more about a seagull pooping on it.