Nothing, any action you take will blow up in your face. You see nothing, know nothing, do nothing. At the end whatever way the ball drops you treat both parties equally the same. It is their life, not yours period.
Offer no opinions. Do not out the parties. Listen, however take no action. You may want to get involved, do not.
In all things you are Switzerland, Neutral. Anything else may bite you in the Ass, probability is very high