I don't read it that way. I think you're confusing person
(who's telling the story) with perspective
(who the story is tracking). Usually those will be aligned, but in this story they're not.
Chapter 1 is all first person from Brittany's perspective.
Chapter 2 is still first person, and although it's following Chelsea, it's still narrated by Brittany. It would be easy to mistake it for third-person since she's not actually present for the events she describes, but wording like "After dividing up our targets" and "Technically, I was dressed exactly like a cheerleader..." identify it as first-person and the context of the second establishes that Brittany is still the narrator.
IMHO this is not a great stylistic choice, because it leaves the reader wondering how Brittany can provide such a detailed account of something she didn't witness.
Chapter 3: first person, still Brittany, this time describing her own role in things. It does shift from past to present tense in the middle, which I found annoying.
Chapter 4: still first person, following Samantha this time but still Brittany narrating. ("Just as I returned to Chelsea's home, Samantha was preparing for her day..." and "I have never done it, or had it done to me, but she swears by it...") Again, random tense-shift (past, present, then back to past).
The epilogue is in third person, but it's very short. This is the only person-shift in the book, and for my money it doesn't really work. It's not unusual to switch to third person for an epilogue that goes beyond what the main narrator could have observed - but we didn't do this for chapters 2 and 4, so why should we do it now when we're describing events Brittany actually could
For my money, this story would've worked better with third person throughout. Alternately, keep chapters 1-4 in first person, but switch narrators in each chapter so Samantha and Chelsea tell their parts of the story.