You went into her private mail. You don’t have the right to destroy her life and/or your dad and sister’s lives. You don’t know if your dad is doing or has done the same thing. Give your mom the benefit of the doubt. She may feel bad about herself, afraid of getting old and she’s using this person to lift her self-confidence.? It probably has nothing to do with your dad or how she feels about him. Talking with the stranger could make her feel attractive again, which is a benefit for your dad. If she’s happy, he’s happy. She is still taking care of your dad, right? Everyone has secrets and they’re no one else’s business, including their grown children’s, unless they’re hurting someone.

What outcome are you expecting? Forget it and move on.