My first guess, is that there are multiple people involved (not necessarily there when "it" happened, but have knowledge/a hand in covering something up) and they were people or a person she was acquainted with and semi-comfortable to leave with.
However, I'm not discounting the possibility that this was a person she didn't really know and/or had maybe literally just met the night. I see it many times, people out at bars or party's, they meet somebody throughout the course of the night, strike up friendly, casual conversation with, think they are a nice person, etc. Through a series of random and innocent events, they get separated from friends, lose/forget a phone somehow and at the end of the night, you're tired, a bit buzzed/drunk possibly, maybe a bit annoyed and angry that you lost your ride or your ride forgot/left without you. At that point, you just WANT to go home and leave. So you bump into "random nice guy" again or he sees you're by yourself and he offers or you ask for ride home, thinking nothing bad will happen, because you're being seen by a bunch of people. Unfortunately, that's where things take a turn for the worse asit did with Chelsea.
Just something that should still be considered.
I think in regards to the costume being found separately from the body, it could be something as simple as the guy(s) forgot they had it with them or in their car after the fact and on the way home or where ever, they stopped off at secluded enough location and just drop it off there and they continue on their way thinking there gotten out of their crime.