Suzy is at Mary's house, they're getting ready to go out for the evening. Mary is in the bathroom primping; Suzy needs to borrow a blue hair scrunchie to match her outfit.
"There are a bunch of hair doodles in my room, in that drawer under the book case," Mary calls over her shoulder.
Suzy finds the drawer in Suzy's room and starts digging around in the barrettes, hair ribbons, headbands and scrunchies. Near the bottom she finds Mary's diary. Curiosity overcomes her and she starts peeking.
(She finds the secret, they have their argument, etc)
Twist could be that, although Mary insists nothing happened and she was just drunk, she doesn't realize how drunk she was, and obviously something did happen at the party, because she's now pregnant.
Just a thought or two. Good luck with your story!