You need to stay in reality here. She at a minimum had an emotional affair, but she is confused. I wish I knew the right answer, you can't really stalk her and keep her away from the guy, but you can let her know that you want to work on the marriage, she is important to you and if you both focus on EACH OTHER, then there is a chance to save the marriage. KGRow may be right in that it was her sickness that triggered the mid-life crisis, and the fact that you were there holding her hand despite what you must have been feeling speaks volumes. So without invading her space let her know that this is a time for healing both physically and emotionally and most of all I'm glad the surgery went so well.

Good luck,