Cassie watched her thrash around on the bathtub and honestly, she admired Arianna's courage to go through with her plan of suicide. Although she was surprised what might have triggered that decision, nonetheless, everything worked in her favor or so she thought.
It wasn't until Cassie realized that Big Joe would be angry if anything were to happen to Arianna and that she would be the first suspect in this case, made her finally make a move. Even if it were a suicide, Big Joe wouldn't believe her and would use her as a scapegoat to appease his anger - Arianna committing suicide days before her marriage was a big slap to his face and a huge stain on his reputation.
So Cassie went into action immediately, pulling her naked form out from the tub and began to perform emergency