Well, there is one way out... he quits being Jedi alltogether, and find a way of her own... nope, no darkside, no heroic death, no mysterious disappearance. Then it would make sense she is not mentioned, better if she lost her connection to the force in some way. She may be missed a bit, but not too much since she is still alive and well and having a decent time as a normal person. That would also spark Anakin's wish of having a normal live with Padme (although he would want to have the cake without quitting the jedi order)
It would be much easier for her to sneak away from order 66 since she is not a jedi at the time. If she is smart enogh to be on the move once3 sheet hits the fence there is a high chance she is not tracked down by the Empire, and Vader not tracking her down with effort is also reasonable, he still has his inner struggle (and easily explainable if Asoka lost her force connection).
Now that is one way of fading away a character, without making her into something great, no lightside or darkside, just poof.