Should I Do Anything Extra to Protect IPhone After Going to Beach? My friend told me that the salt in the air on the beach will reduce the lifespan of iPhone. 

Is it true? If yes, is there anything I can do to "clean up" the salt in my iPhone?
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.