Just to add my opinion--I think quizivex is right that "working on a paper" is not worth mentioning and also that there is little difference between "in prep" and "submitted". It's true that your prof ideally should only let you work on writing a paper up or submit a paper if it's actually ready to go, but the committee does not always know this. Also not every prof works like this.
If this was the application submission stage, I would say generally, I would only include a paper in the application if it was at least "submitted", not if it was "in prep." But since the application is already sent in, I would only update the committee if the paper status became "accepted".
Also to respond to the user "ol" above, I think that even keeping in mind how ApJ operates (I'm in astronomy/planetary science), I would consider the first referee report to be the first significant hurdle in getting a paper published. If the report is at least "recommend for publication pending the following changes" then I would agree that it's basically just a matter of time/effort to get it published.