The more typical, effective, and elegant way of phrasing the original sentence is, "Do all sentences tell us what the speaker wishes would happen?" (Kristina is correct that the original is grammatical, if clunky.)
Adding "for it" to the original is simply wrong because it inserts an unconnected and unnecessary word pair in a place where it could not possibly have any syntactical validity or purpose. "Like" and "for" can not be put together this way. "Like" is a transitive verb that takes a direct object. It can be followed by "for" only if using that word to mean "on behalf of," in this way:
What I would like for the world is everlasting peace.
But that's not the meaning intended by your friend's suggested correction. Your friend's suggestion is an attempt to insert the direct object of "like" (while misusing "for"); he's trying to say "like it to happen," with "it" becoming the direct object. This, however, ignores the fact that the word "what" is already present in the sentence as the direct object "of like." Inserting "it" would therefore be redundant and unnecessary, and hence incorrect.