If you genuinely believe that she wouldn't play with your feelings then by all means, contact her. Even if the only thing you recieve is the cold shoulder, it will provide some sort of closure (but you don't seem the type to need the traditional type)
Speaking from experience (not experience of cheating or anything, simply a bad breakup) she probably misses contact with you, even just a little, if you were as close as you imply. You sound extremely mature about this, letting it all be water under the bridge, so the possibility of rekindling a friendship is possible if thats what you want
I wouldn't go down the path of an "interrogation" of either her, or your friend. Ask politely, with reason, and if they feel they don't need to explain, let the subject drop and move on
Worst case scenario? They both cold shoulder you and you can move on with absolutely no mental or emotional damage