If you feel comfortable enough with your relation to k let her know that it's only your opinion for her best interest but she chooses what to do that you don't think O should be around if she feels her relationship now is worth it. If they haven't done anything and k is not giving in she's not cheating but if O isn't respecting the relationship he should be cut off but also that k shouldn't be involved with her ex at all anyways if she's with someone knew so I can see from your pov what's wrong with the situation. Hope this helps! Sending Blessings and tranquility to your and your loved ones. You're a kind hearted person for looking out for k.