Why did Ro ..... SPOILER

Grace had dark shades. She was blackmailing Hill too, that she would expose him to his wife if he did not continue meeting her. And she wanted Ro to go to print with the story of Hill the big ad-man and his secret love nest. Because that was what Ro did - She wrote daring stories. The details Grace gave her, set the premise for something Ro could hold against Hill. She could make it look as if Hill did away with Grace because of the blackmail. She's a journalist, hence she was good at research. Some look at Hill's company, and she found his wife connected to the extensive use of the poison in a photography studio - Series on dilated eyes. Which would mean Hill had access to the poison. Ro used this premise to frame Hill. She needed to frame someone. At the very end, when she's showing the 'chat' with Hill to the cop, she establishes the fact that Hill chased women...And Grace's blackmail would seem as if Hill had motive to do away with her. If Grace went to his wife, he loses all the money and would end up with a nasty lawsuit. It would have left him bankrupt. Ro put everything together. She was just good at investigative reporting, hence.