Why does my son have nightmares? why won't he let me comfort him? need expert advice!?

ever since my son's BFF has moved in with us ( the poor kid had a ruthless mother) my son have been having night terrors! late at night i can hear my son crying and thrashing and moaning in terror. when i go to the boy's bedroom, the door is locked and my son screams for me to go away. i have tried talking to his BFF about it and he tells me..."don't worry about it, i'll help him with this". i'm afraid this will affect my son's work performance...he was just promoted to crew leader at McDonalds...a great accomplishment for a kid of 23. should i boot the BFF out or take him to therapy? the boys have been BFF's for over ten years now and are inseparable. any parents have any advice? thank you so much in advance.