I do think it started as only that. I then think it progressed to flirting, and this past week I do believe it has progressed further than that into a full-blown EA. I do not think it's a PA yet, but I could always be wrong, and it doesn't change my course-of-action anyway.
It's hard to decipher the text messages, as it's difficult to see which one of them said what, and which is the reply. It does seem like he's doing all the pursuing, but that she is also encouraging it. I would say the worst ones have been "Do you miss me?", which I'm certain they BOTH said, and "Will I see you Saturday?" (he trains her -- INSERT BAD JOKE HERE).
There's also of course just a lot of what they are BOTH trying to rationalize to themselves as just "emotional support," and encouragement. But I've never met a man yet that didn't want to escalate that, especially if you lead him on, and at THE VERY LEAST, she has led him on, flirted, and not set any boundaries for him.
So I'm assuming the worst. If it turns out better than that, then "gravy."