As the parent of an adult AS, I can only speak to what would be upsetting to me. And I could only think of two things.....and that is if the girlfriend was an active addict or fiercely codependent. Either of those two situations would be detrimental to his son's recovery from addiction.
If your bf's situation is anything at all like ours, his father may just be sick and tired of the behaviors that are associated with active addiction. Tolerance to just about anything related to drugs is low at best. His sense of humor may be tapped. His finances may be tapped. His nerves may be tapped. And his fear and anxiety may be soaring through the roof.
Your bf's relationship with his father is probably extremely strained. Drug addiction does that to families. And sometimes the best reaction to someone is no reaction at all.
Wishing the best for your boyfriend's recovery. You'll all be in my prayers.
gentle hugs
ke