Lea, tell the MIL, she deserves to know. I would tell her only the facts as you know them and allow her to fill any gaps on her own or through questioning her son. As usual expect the blowback from H, as he will be ashamed from the heat he will get from mom.
Its difficult to be or see the positive, but at least your H considered your son request. My guess he wants to discuss it with the OW first, trying to soften the blow for her, cause I'm sure she will not be pleased the son doesn't want her around.
DO ALL YOU CAN to protect and encourage your sons to be as positive as possible in a bad situation. They will respond to you emotionally, as all they want to do is protect you.
