Am I the 3rd wheel in my friends marriage?

I am friends with my best friend over 26 years.I was by her side when she got married 4 years ago and always got along with her hubby and was there for them both as much as I possibly could be. Me and her are very close and do spend a lot of time together. So the last year or so they have being goin through a rocky patch and ending up having to get marriage counselling. Last week I got a text off her hubby basically calling me a third wheel in their marriage and if they are to make their marriage work I am to stop spending time with her. I was absolutely devastated by this as I feel I am not to blame in this situation. I told her about the txt and she is really annoyed at him for sending me that. Now what should I do?? Should I step back from her?? I think I am totally finished with him as I don’t like the way he has been treating her lately. My friend thinks he has crossed the line. I’m so hurt by all this, please help xx