My best friend's husband hates her having single friends!?

Your friends husband sounds very insecure. He may be afraid that while his wife is hanging out with single women she is acting like one and not thinking about him. He may also fear that she is cheating on him. While I agree with you that he should not boss her around like that I wouldn't suggest saying anything to him. I would talk to her and see what she thinks about it and give her support and courage to speak with him about it.