I had a close friend like that and at first refused to see it in her. We would go out after work and she would always hook with some guy in the first five minutes. I finally figured it out that she needed me to be with her, since she really did not want to walk into the club alone. I'd go home and later she would come back to the apt - she lived across the hall from me - with the flavor of the night in tow. Still figured it was her life and her business. But I did tell her that being associated with her, when she did that, was not comfortable for me. We could do anything else; go out to eat, go shopping, go swimming, a dozen other things. But we were not clubbing.
You can be assertive without being aggressive.