Choosing to rat or not is irrelevant.. The cop's window of opportunity to charge you is gone.. They're free to lie to you and tell you you might still face charges since they have the cell#, but it would be a lie..
My advice if you hear from them again, be polite because the last thing you want is for them to take offense and make a hobby of harassing you, but keep in mind you're free & clear by the book..
Just thank them for the call, and say that if you learn anything that should rightfully be reported that you will call them personally.. Don't speak to them about 'anything'! If you incriminate yourself on any matter then everything I've said so far is null/void.. And DRILL this into your slow friend's head!! If they do end up going for further incrimination you can be damn sure they'll try to get you both involved..
They may even be decent cops.. They did let you avoid a bigass legal mess, and you can't really blame them for wanting intel.. Maybe some punk will go for their story and give them something that should righteously be stopped and makes their career so they don't need to lie to kids anymore..