This one is mostly correct in terms of grammar and punctuation, but there’s a bit of confusion with pronouns (which pronoun belongs to which antecedent) and more than a bit of redundancy.
I’d recommend changing befriends to pretends to befriend in the first sentence. I’d also recommend changing the latter part of the sentence to plans to take him on a camping trip and abduct him. Neither of these changes is necessary, but they would make the sentence less clunky and eliminate the use of he/him referring to more than one person in a single sentence.
I’d recommend deleting he befriended from the second sentence; we already know which human is being referred to.
I’d recommend changing Unknown in the second sentence to Unbeknownst (even though it’s a “big word” of which Grammarly disapproves — unless you’re new here, you already know my opinion of Grammarly’s ridiculous ‘these words are too big for real people’ list). Another option would be to change the sentence so neither word is needed.
I would strongly recommend deleting the last sentence entirely, because the reader was already informed that neither character knows about the other’s plans. (If the author insists on keeping the last sentence, other’s needs an apostrophe.)
An alien, disguised as a human, pretends to befriend a human and plans to take him on a camping trip and abduct him.
Unbeknownst to the alien, the human is actually a government agent who plans to capture the alien on said camping trip.
(Why is it abduction when an alien does it to a human, but it’s capture when a human does it to an alien? Mercenary Proofreader does not approve of species-ist biases… Not that you care, but it needed to be said.)