Ok, but why wouldn't CH call for ambulance? He knows he has to get to hospital no getting around that, why wouldn't he call the cops either? The seriousness of his injury has got to outweigh his possible probation violation in the passenger scenario. She called and for some reason she hung up. She was probably the more prudent of the three but she was convinced to hang up.
JMO of possible events
For whatever reason CH is driving, doesn't know the road and crashes. Breaks ankle. Maybe there is Meth around the area or CN knows he is in deep because of car. (Maybe after the wreck the car was primary concern for CN and decided to go with a "stolen" car scenario) They can't be found near car so CN carries CH for a while piggy back. CN is lost too, the woods are so different at night I can't even explain it to those that don't know) Nothing is familiar. They try this for two hours, even more lost and now CN is exhausted. (The 911 call).CN convinces her he can make it home, CH doesn't want to get in trouble, she hangs up. CN strikes out for help alone, after several hours MM is thinking now and is really worried about CH, the "upset" phone call. CH dies early morning, MM strikes out on her own, CH hasn't returned yet. CN makes it home, returns to help, and finds CH deceased where he left him. Where's MM? He drives the roads and finds her, now he has a big problem. (1) dead and (1) witness. Maybe there were drugs involved, maybe that was why no cops were called. Either way, after the wreck they are all together. CN doesn't leave them to go home immediately, they would have called friends much sooner. CN is with them in the creek bed I think. If he is intent on doing harm to these two, he would have found them sooner and they would have made references about him looking for them. CH's death was the catalyst. When that happened CYA mode kicked in and MM was a liability. It was not well thought out, it would never have happened if he realized the phone calls were made. The cover up was planned. Anyway, just my thoughts.
