Well I'm 26 and we all know my latest story. It sounds somewhat similar, although it wasn't about her ex. It's just a matter of investing in someone you're not sure is willing to do the same, and if that's the case fuuuck that dude. If she comes around and says she wants to be serious, then sure go for it. But don't invest time and energy in someone hoping they'll return the gesture, it's a recipe for disaster. Women are just like men, if she's interested you'll know. Until then, keep it casual, enjoy fucking when you do, and go about your life.
As Haast said, I understand it's hard to not get invested. But it'll be harder to get over that shit if she burns you.