All I recall is that the detective was talking to a crime boss outside at a table in a garden and the crime boss suddenly ordered one of his henchmen to punch his own (the crime boss's) lovely young wife or girlfriend in the face. It was an American film probably made in the early 1970s.
sharePoint Blank https://www.imdb.com/title/tt0062138/ popped into my mind when I read your description. I'm not 100% certain though. Lee Marvin (not a detective) with Angie Dickinson as the woman and Carroll O'Connor as crime boss.
shareNo, it was not Point Blank. I remember that scene with Lee Marvin and Angie Dickinson, and it was Angie punching Lee while he just stood there like it was nothing. In the movie I am trying to recall, the private eye is at a meeting with the crime boss and his henchmen, and the boss has one of them punch his own girlfriend out to make a point. I don't remember if she had been fooling around with the detective, but I think he just did it for no reason other than to intimidate the detective.
shareIt’s not outside and it’s not done by a a henchman, but there are henchmen and a private eye present when a crime boss bottles his own girlfriend in The Long Goodbye. You can watch that scene here:
https://m.youtube.com/watch?v=7t7gG3XVqW0