dominique johnson , 19 , was arrested on wednesday and has been charged with conspiracy to commit armed robbery .
she is the roommate and alleged girlfriend of jamyra gallmon , 21 , who last week was charged with first-degree murder .
david messerschmitt , 30 , was stabbed to death on february 9 at the boutique donovan hotel in washington d.c.
gallmon has admitted to police that she set up the lawyer so that he was expecting to meet a man for gay sex at the hotel .
when she attempted to rob him they got into a struggle and she pulled a knife from her pants and stabbed him repeatedly .