Getting flashbacks reading your post, I had nearly the same situation a while back. It's a good sign I think that she hasn't already reached out to your main (framing it as "helping her avoid a terrible man" or whatever). If she does, iron frame, admit no wrongdoing.
What I did when mine confronted me was give a stalker spiel, mostly true but leaving out large chunks. Then I said something like "I understand that you'd be suspicious, anyone would be, so go ahead, you can ask me anything that's on your mind. But this is the only time, you will not bring this up again, I'm not going to deal with this being brought up over and over since I've actually done nothing wrong."
She bought it but was still suspicious, only ended a few months later when the other girl got the messages off the cloud and texted the main (the night before valentines day, but yours sound more naive than malicious).
What I did when mine confronted me was give a stalker spiel, mostly true but leaving out large chunks. Then I said something like "I understand that you'd be suspicious, anyone would be, so go ahead, you can ask me anything that's on your mind. But this is the only time, you will not bring this up again, I'm not going to deal with this being brought up over and over since I've actually done nothing wrong."
She bought it but was still suspicious, only ended a few months later when the other girl got the messages off the cloud and texted the main (the night before valentines day, but yours sound more naive than malicious).