Nikki and Gus
I was surprised that Nikki and Gus didn't say anything to Sarah's mom. I would have, after I saw the dance incident at least.
Even if Sarah had said she was breaking up with Rob, I still wouldn't take chances.
They DEFINITELY should have said something before Sarah was missing!
The fact that Nikki didn't do more to help her friend must be eating her alive to this day :(