DO NOT REPLY. Call her. When you do talk to her, keep it simple, and don't exaggerate what you know. Your purpose is to let her know that your wife is having at least an emotional affair with her husband, and that you thought she should know. You can "compare notes," and can continue to do occasionally, but that you have no desire to create a bunch of "drama" or to invade her privacy on a daily basis. Give her your number, and tell her she can call you if she wishes to or if she has some important information she feels you would need to know.
Be kind and compassionate, and thank her for agreeing to talk to you.
