I go in between thinking he was drunk and wandered off and came to his death due to water/weather OR that the friend did something. I agree with you on the above, it surprises me he went with them, but, if I remember correctly, he was not staying at the hotel overnight. I THINK (again it's been awhile) that he lived close and ended up going home, even knowing Tyler still had not returned. I've been at places in my relationship where a "third wheel" was nice because we weren't getting along super great and they broke up that tension. Totally reading into things here, that may not be the case at all and they may have just met up with a friend they don't get to see often.
The crime side of me understands why this friend is staying out of the public eye, but then friend side of me doesn't understand.....I feel like this one along with the Brandon Lawson case has a lot of details missing. Probably to protect their images, but it's hard to put all of the pieces together when you don't know.