From what I have seen it is either repeating what he saw his mother getting treated like when he was a kid or he is showing his dominance over his Mom to others to make up for something. Maybe financial dependence? Maybe a failed relationship he is blaming his Mom for? Who know. Have you asked him?
Who knows. He could be on drugs/alcohol, or coming down off drugs/alcohol. He might just not like his mum. He might get a kick out of it. He might be jealous of his mother having friends and company. Your question's a little vague. If I were her, I would just ask him about it when company is not around. Say "whats up with that son? Why are you saying/doing those things when __________ is here?". At least she'll have some idea of what's going on.