Bay, how would you feel if you were the one being deceived and not told?
An old friend of mine saw my ex-husband at a Celtics game with with his brother and two other women. His brother had just recently split from his wife. It was visually obvious that women were dates of my then husband and his brother as seen by my friend. She didn't tell me until me and my ex split up (he left for a 20 year old...who in the end left him right after we split - gotta love that ). I asked her why she didn't tell me...she said she didn't feel it was her place. Can you imagine hearing that?? It wasn't her place to tell her friend that her husband was cheating on her?? How could you look in the eyes of a friend and know they are being deceived by the one they love? Not to mention possibly let the cheater infect their friend with some potential disease? Not telling (IMO) is ignorant and possibly dangerous.
Obviously, I would not be happy if I were the one being deceived and not told, but the issue is between my partner and me--not between my friend and me.
I certainly understand why you are upset, but I must agree with your friend. If she feels it was not her place to say something then it was indeed not her place. These are individual judgment calls. What your friend may have been saying is that she didn’t feel as close to you as you did to her. Accordingly, her decision to effectively mind her own business was justified. She might have made a different decision with a different friend.
As I said previously, whether or not I would speak up would depend greatly on how close I felt to my friend.
In addition, things are not always what they seem. Just because you see two people together on what looks like a date—even if they hold hands and kiss--does not necessarily mean what you think it means. It’s not your friend’s job to play detective or otherwise “figure out” what is going on because at the end of the day it is not her business.
The person who betrayed you (and the only one you should have issues with) is your ex—not your friend and not the person your ex was having the affair with. Your ex put your friend in the awkward position. Don’t punish your friend for that.