Return it, or even better, leave it some place where she can find it without knowing how it got there or who returned it. If she knows you found it, that might leave a doubt in her mind about whether you read it or not.
Never mention it, don't read it. That can only open doors that could eventually put your friendship in jeopardy.
The right thing to do is to just not go through it and just give it back.
I respect my friend's privacy and if someone found my diary I'd want them to do the same.
Nope. Unless I didn't know who it belonged to and had to peek on the covers. But I wouldn't look at anything else. I don't think I could call myself a friend if I did something like that. I'd feel so guilty. =p
I'd rather not do something and then regret what I did later. Although I have to admit that I would wonder about the diary's contents but my loyalty surpasses my need to worry about something like that. I respect privacy very much in friendships. Although when it comes to close friends, there really isn't anything I'd hide.
put it on face book
I would not even open it or read the contents.
Return and just ask if I want to kno.