Sweetbriar, I am 100% in agreement with greatwhitenorth...you need to speak to a children's counselor to see how best to proceed in working w/your daughter on her drug use as well as any other experimentations.
Your daughter may have been leaving the journel out to see if you would read it. My advice is to tread this walk very slowly and gently. She's very vulnerable right now and is emotionally raw from what has happened.
The first step is to seek out a counselor, speak very openly w/him/her and see what they suggest. This should be your number one priority. Your h and his visitation arrangements cn go to h@ll for now. He lost all rights to what he thinks he can control and manipulate the day he walked out the door.
Keep your focus on you and your daughters. They need your now more than ever.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.