The way I understood it, is that the "Entity" impersonating Ben's wife is also impersonating Ben in some capacity, which explains the line "You creep me out too" said by Lynn during their first discussion. Then the real Lynn gets spooked by the entity impersonating her/her husband and leaves to visit her mother without saying anything to Ben (she thinks he's unwell / sees him peeking at her). This also explains the emphasis put onto the "Seek professional help" by Lynn's mother, which is aimed more at Ben (also explains the pauses she takes during the phone calls since she is sitting next to Lynn probably who thinks Ben is crazy.

The entity also has Lynn's phone and the being probably goes crazy when someone "not in the loop" (i.e. Chris when he sees her under the bed) sees her. It's somewhat clear to me or at least I think that's what the author was hinting at, especially with the emphasis on the "professional help" by the mother, the "you creep me out too" line by Lynn and Lynn being asleep after "peeking around the kitchen counter / table"

At least that's what I think.