When a best friend dies and returns as a ghost, a human girl must determine if her best friend is who she says she is.

A: The term, ‘human girl’ is awkward, I would drop it completely. (Perhaps give us a personality trait)
B: You should say, ‘her best friend,’ instead of ‘a best friend’ (It’s more personal)
C: You don’t have to tell us the friend dies and returns as a ghost, since being a ghost implies the best friend is dead.

However, despite my nitpicks, the logline isn’t that far off. Not bad.
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“When her best friend returns as a ghost, a (lonely college student) must determine if her incorporeal best friend is who she says she is.”
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btw I put ‘lonely college student’ in parentheses, but if the lead character is a high school student, or a banker, or she is ‘enthusiastic’ replace my example with whatever traits best describe the character.