As much as I would like to say "no never". I think there may be times when a parent should. If my child had been dishonest and uncommunicative with me, I think I would read the last three entries of her journal to find the answer to the questions I'd already been asking. I would never tell her I did. I would lie and say your friend told me.
If I noticed changes with my child I wouldn't head for her journal, that's violating her privacy and will shut her out further from me. But if I she was lying to me, showing dangerous behaviours, dissociative and not answering any questions I asked her, I would. I'd hate for my daughter to think that she would have to suffer anything alone. But that's just me.