PETER AND SARAH WERE IN HAWAII AT THE SAME TIME AND AT THE SAME RESORT BY COINCIDENCE
The trip to Hawaii was instigated by Sarah breaking up with Peter. He was still having a hard time in Los Angeles trying to get over their break-up. Peter was reminded of her everywhere he turned according to the conversation that he has with Brian, his step-brother:
BRIAN: What are you burning, man? Come on.
PETER: It's everything. It's everything that reminds me of her...and I need it gone. Okay, I thought I was fine, Brian,and I'm
not. I'm not fine at all.
BRIAN: Okay, let's just put the picture down.
Brian is the one that suggests that Peter go on a vacation to get away from Los Angeles. Peter suggests Hawaii because there was a place that Sarah was always talking about in Hawaii:
BRIAN: you know what, go on a vacation? Go to the Alps.
BRIAN: Go to Gstaad. It's the best.
PETER: I could go to Hawaii. Sarah was always talking about this
place...in Hawaii that was supposed to be nice.
So, Sarah was the reason that Peter made the decision to go to Hawaii, but he did not know that she was going to be in Hawaii at that particular time (especially with another man so soon after their break-up). He also was unaware that she would be at that particular resort. Both Sarah and Peter are in show business and that resort caters to more elite guests.
It was a coincidence that Peter and Sarah happened to be in Hawaii at the same time as evidence from the conversation between Peter and Sarah when they see each other at dinner:
SARAH: Look, did you follow me here? Did my assistant tell you I was
SARAH: Did you talk to her?
PETER: No, I didn't talk to your assistant. It's not all about you,
you know. Hawaii is a beautiful place. People come here, usually not
to follow you.
The only real set up to them being at the same resort would be the fact that Sarah had previously mentioned to Peter about the place that Sarah was always talking about that was supposed to be nice.