Circumstances have lead me to witness intimacy between a lower rank female and the Master. They are both married. I know the woman and she is indeed in a bad marriage and extremely vulnerable. I am very conflicted by the whole thing and would like suggestions as to how to handle this.
Well, respectfully: I disagree with Jim (please don't hurt me, Jim
Certainly, it is a difficult situation. There is much potential for abuse in an extra-marital relationship btw Sensei and student. And yes: I would personally call the Sensei's behavior irresponsible (definitely, outside the bounds of good etiquette, in many dojo's).
But "un-Aiki?" I have always been troubled by that term. And I think that while there is a good chance that your Sensei could be abusing his position: I would tread carefully, in casting about such labels or throwing my weight into situations where my opinions are unwanted, or unhelpful.
If you're the friend of the woman: I might talk it out with her; but IMO, tread carefully when broaching private matters with Sensei.