In my opinion telling the OM's wife is morally imperative. By not informing her you are keeping a secret from her and are actively protecting the affair. OMW deserves to know what is happening to her life and to make her own choices accordingly. You telling her the truth won't hurt her, her husband's actions have already done that.
And as aside, in my experience the vast majority of the time when caught an OM will run back to his wife and cutout without a second thought his "side action", which happens to be your wife. Conversely, I have observed that this is not true of most WW. Some will drop their affair when caught, but most have developed a strong emotional bond to their affair partner and will only consider stopping an affair when they become overwhelmed with negative consequences. Moving out is a negative consequence, but moving out temporarily to give hubby some space and using this as an opportunity for more time with OM is definitely not a negative consequence.
