The thing that ur fren cant join u until final cutscene is kinda bullshit. U wanna adventure with ur fren but ur fren need wait u like a sohai outside until final cutscene. If starting game cannot play with fren, fren oledi lose interest how.. stupid move mhw
hunt a few more times, you'll start to be able to predict his moves then it will be a walk in the park
also prevent him to go back his lair to sleep, most of the casualties happens there flash him or shoot him with dragon pod when he's limping back, high chance you'll provoke him to return to attack you
also when he's at the big area with crystal overhangs, shoot those crystal down when he's below for free massive 1K damages
did u joined random session? there is possibility that he joined your quest through same session.
like the rest said, put restriction to 1 player only.
or, like i always did, SOSed, if they died one time, can be he died by unavoidable circumstances for example stun and nobody near him trying to help him. After that i will inspect their equipment first, if died again, will kick from quest. heheeh
ohh if player list i owes can see is player list for the online session.. not in the quest one.. let me try again next time lol.. sometimes really got ppl come in and screw up ur quest.