Reading someone’s diary is taking something they never offered. Close it.

A private page is somebody working out how they feel with no audience, which is the only place most people manage to be honest. Reading it takes the unedited version of a person without asking for it. You cannot hand it back either, because now you know and they do not know you know. Curiosity is understandable and it is not permission.

Close it, put it back where it was, and do not mention it. If you are worried about them, say so to their face or to an adult. Worry is a reason to ask.