I think you two have different notions of what proofreading implies.
The first aspect is grammatical validity, the second is semantic validity, and the third is linguistic coherence. The first part is as easy as plugging the text into MS word and looking for the red lines, the second (as QuestionableNinja mentions) revolves around the text making sense to the reader within the context it is presented (such as MC pointing out that he can hear steps behind him when listening for the woman inside the house), and the third (also what QuestionableNinja points out) is often referred to as whether the text appears 'organic' or 'stilted/wooden/expository'.
Two close friends would likely not explain everything they see and think when meeting each other ("Hello good friend, I am glad that I have known you for 10 years such that we can speak to one another in a way that could, for strangers, imply that we are enemies to some degree. How are you now that you have left the house and accidentally wandered over to the same mall where I am currently engaged in purchasing new clothes for my upcoming adventure that will soon occurr"). If you met a close friend, chances are that you would just say "hey bitch, what up?" and altogether skip the summary of your last five years.
Within that context, S1 may well have been worse grammatically, but the lack of an existing history between MC and the other characters (that we have been party to) does make the conversations between them appear more reasonable, something that cannot be said for the subsequent seasons. Once you have rawdogged a woman some 40 times (such as with the step-sister) it becomes awkward for everyone when you talk to her as if she was a recent acquaintance. Moreover
it annoys the reader as it somehow eradicates what the reader has experienced, affecting immersion negatively when nothing seems to matter.
