Although walking away would be the easiest thing to do, if she has interest in you and you have interest in her, try and spend time with her, but remember you are not a couple, so in her mind "getting it on" with another guy isn't much of your business. Though, try not to allow yourself to get attached unless you're sure there is a more serious relationship in your future, then I would suggest letting her know how you feel about her if you haven't already. See if she returns the feelings, and if she does, by this point she has most likely stopped "getting it on" with the other guy if she really cares for you. Though, I know from personal experience that isn't always the case, and the best thing to do in that situation is just let her know you cannot be in a relationship with her if she continues her relationship ("getting it on") with her ex. This may sound obvious, but sometimes people have to be reminded of these things.
Someone said above to just be forthright with her, and I agree, although I would do it over time. Do not play games or allow yourself to be hurt by being honest with her and asking her to be honest with you. That's all you can really do.
Scott