Well, the getbigger in me is saying you should tear shit up all day no condom no pull out, but if you were my friend in real life I'd say don't do it. Too much drama can come from sleeping with an ex-gf. You'd be much better off just finding a new fwb to fuck around with.
If you do end up hooking up with her (which I'm sure you will, HAHA!) you better tell her right away that you guys are just fucking, and there will be nothing to come from it.