What is the best thing to say?

I will try and keep this brief. Many years ago one of my best friends had an affair which resulted in her divorce from an awful violent man. She eventually married the man she had the affair with and he became very controlling and was awful to her with gas lighting and coercive behaviour. She eventually got him to leave and started divorce proceedings. She had to get the police involved as she was scared of him. He met another woman and then she did not often hear from him. A couple of months ago he turned up with a problem with a large swelling down below. Her being the kind natured person that she is she went with him to the Hospital and then to have tests and GP appointments. His new relationship had failed and he had got some of the bad treatment himself. Now he rings her all the time, will turn up, and quite often they will go out for the day. I popped round to see her, he was there and he made snide comments. I do not understand why she can let this man back into her life when he treated her so badly. I know it is not my choice but he is awful. Should I just leave her be and pick up the pieces when it happens again, or should I tell what I am thinking. Probably not my business but I am worried about her.