Without the attached picture, it's a bit challenging to provide precise guidance. However, based on your description, if you want to convey that someone or something stopped behind someone's back, you would typically use "behind" rather than "before." "Before" typically implies something in front of or ahead of something else. "Behind" indicates the position at the rear or back of something. So, for your sentence, if you want to convey that someone or something stopped at the back of the person in the scene, you would use: "They stopped behind his back." This would indicate that whatever stopped is situated at the rear or behind the individual in the scene.

Your intended meaning isn’t at all clear. Are you talking just about the location, or are you also giving information about the orientation of you both? Some examples of possibilities: They stopped behind him. They stopped 5 meters behind him. They stopped behind him, out of view. They stopped behind him, facing his back. They stopped 10 meters away from him and were careful not to be visible to him.

Behind is the correct choice indeed.

Why would you identify somebody’s location by the location of their back? It doesn’t make sense. ‘Behind someone’s back’ is usually said to convey that the person isn’t aware of what you are doing.

“They stopped behind him.” Say no more. The mention of his back only confuses. The word “behind” already means “behind his back “.