when your friend’s boyfriend is overly controlling

I have come to learn a few things about relationships in my years. First and foremost is that if a guy is overly controlling then he probably is also an abuser. It has become very apparent in the past few weeks to me that my best friend’s boyfriend is overly controlling, to the point of attempting to isolate her from certain people. I am not okay with this at all. I also don’t really know what to do about it either. He is a very good guy, except for this one thing, and my best friend is madly in love with him. Is there any way that would be appropriate to address this without questioning her decisions? Or am I destined to sound like a bitch because I said something?