Respectfully snipped by me.
Text messages can be saved and locked so that they don't delete with other messages. I have personally received text messages that I wanted for proof later on and hit the lock button.
It's possible she instigated a conversation getting him to talk about this or make comments and was saving them in hopes to turn him in or prove to someone, even mom, that it happened.
It also could have been something like someone provided a little alcohol and things got out of hand. At the time MC might not have said no, but when she was in the right state of mind she realized what happened wasn't ok. She could have been taken advantage of. Being a minor charges could have been pressed, even if she said yes. If it was agreed upon by both it would still be statutory rape. If a teenage girl told someone she confided in she might not use those terms, but just call it rape.
Again all just speculation and thinking out loud.