This may be cruel and blunt for this incident (and feel free to provably slap me for asking since I haven't the full details), but it didn't seem like there was much fight in that altercation. She had an ex and still has "feelings" for them - And...? Is this meant to be the surprise we're all waiting for? Is there an out-right reason why they split? If there was something about that other person that wooed her the 1st time, could it not strike twice, and if so was it worth the split in the 1st place? (Those last few questions are for her to deal with, not you.)
What I can see from this is that you had fun with her, and you liked her company. But was there a proper face-to-face conversation to determine closure and reasoning? None of this text bull - text is for notifications, not discussions. If you chatted when you asked for friendship, I can't say I disapprove as that's a better step. But there's just a lot of mist covering the landscape - nothing clear-cut and resolved.
I dont know what was said between you 2, but I don't think there was enough said from my understanding. It just sounded like there was a bump in the road, and you just looked at it and turned. My response to that text would be to immediately organise a time/place to discuss the issue (if you watched many rom-coms, that seems to be the coffee shop, but it's to your fancy). You attempted it at the start and I'm glad you did. However, when that text came, it sounds like that week of stress weighed heavy on you and it mowed you down when you saw the message. Even as a friend, that's not fair to you, and if there was a strong enough desire to keep that relationship going, then someone should have tried to convince the other that what they were doing was wrong. (In a sense that they gave the wrong answer, rather than immorally wrong.) I'm not saying do something stupid like stalk them, tell them they're degrading or anything like that.
Make a list of pros+cons on your relationship and then show her, tell her the top 5 things/experiences you had between yourselves, tell her how much change there was between before and after you met, serenade, tell her a story - from somewhere or make up one for how the relationship would go (like a plan ahead). These would be options for you, no matter how silly they seemed - Some stupid things are worth doing to prove a point.
You shouldn't of had to be dejected and feel like a failure. I'm not saying she's evil or is a bad person - people make mistakes, and I dont think she fully understood her repercussions. These are things that could of been done, but that may be a ship that's sailed. I'm not going to tell you what to do next - if you are comfortable befriending her, that's your choice and please don't resent her or think of the pain. Forgiveness (if that's the right term for this, if not, understanding) is the best course in this field. Otherwise, do what you think is right and what makes sense to you. Just dont think that everything returns to escapism - you live the life you choose, whether it's active or not. The only way to deal with it is to either face it head on or avoid it - and in both cases, you could be missing out on something. (Either a bullet or an experience.)