Had I known that she knew, then I would not have said anything because then it’s none of my business. “
Though I cannot say 100% for sure what is right or wrong in this case, I will say that I believe that it is especially important when you know someone is fully aware she is doing something wrong, to be mekayeim the mitzvah of hocheach tochiach es amisecha. They might have charata when they see their aveira through someone else’s eyes. other people might feel that if you know they will do it anyway, maybe it’s better not to say something, but that really is only when they are a shogeig (better than being a meizid). This is a toughie.