I don't believe it is Joe either. Yes, he may know where they are, but how would have he got there?
According to the story's time line the day that Gunter, Joe and the Custom's agent are meeting is the day the guys take off in the plane destined for Teluride. This is probably the same day that Dimitri and Mario leave Pacific Palisades to drive there.
The guys have some minor plane trouble on their way , and have a slight detour, but the Air Force whisks them away to their final destination the same day.
Mario and Dimitri drive 24 hours to reach Colorado, so we assume that this chapter takes place the day after the plane incident, so my question would be, how did Joe get there? He really doesn't have much money, he has to check out of his rehab centre, and why would he want to go Teluride in the first place when he can just pick up the phone if he had something to talk to Helen about.