You know what, during your relationship she may have cause to see another man's dick, for whatever reason. The fact that it is your friend's is bad. The fact that she felt the need to comment on it is worse. She did it because she wants to either make you feel badly, and therefore control the relationship with that sort of humiliation, or she wants to fuck your friend... or both. Only a bad person would even mention that she saw his dick, and a worse person would feel the need to degrade you over it. Get rid of that twat (and she deserves to be called that) and find a decent woman who respects you.