I too think it is the fair thing to do for the other betrayed spouse.
I remember a female friend who notified the H of the OW her H was having a PA with. He refused to believe her so she took him for a ride to follow the 2 cheaters during lunch. The poor guy broke down in tears when he saw the truth.
Telling the OM's W will be extremely difficult for you. Be prepared for various reactions from her and approach it with compassion. Tell her your reasoning for getting the A out in the open is to help with its demise.
