What should a wife do if her husband has found a new "friend" at work? I found emails and texts from her.?

She is married, no kids and 16 years younger than me. We have been married 20 years, 3 beautiful children. Tragically, our oldest son was killed in a car accident 2 years ago. It has been so difficult to carry on but we have ..This woman entered my husbands life at work..she said she felt sorry for him after our son died and "tried to help him". She has become a diversion from the pain of missing our son for my husband..I get that on a rational level..and have tried to be understanding. I do not want the rest of our family to fall apart because of our remaining 2 children..they havve already been through a nightmare of losing their big brother and have found the strength to carry on in his honor. I comfronted this woman when I found her texts..told her husband and she promised me it would stop. It did for about 6 months but it has started up again..she said its different now,,they are "just friends" ..she says it is "harmless!! " She has my husband wrapped around her finger..he says he doesn't want to leave us (he did at one time) and she says she is not going to leave her husband. Am I wrong to be upset by all of this? It hurts me to know she is up at work..emailing and calling my husband on a personal level and he is not trying to stop it. I understand co workers can be friends but they have already crossed the line and I am not feeling good about them being up there together. Her husband is more confident I guess. I think she just uses my husband for attention and ego boosts and then goes home to her husband and is happy. She wants to cause conflict in my marriage cause she wants the attention..she has very few friends according to her. My husband sees her as a positive, funny, supportive woman who makes him feel good. I want to be that person for him. What should I do? I am getting tired of feeling second. If we had not lost our son I think I would have asked him to leave so he could figure out what he wants...but I can not do that to our kids..I can not take away the only family we have left but I am hurt that my husband continues to do this. What should I do to try and get my husband back 100%? It seems the less interested I am in him the more interested he is in me, but I am not a game player and none of this is funny. I am reaching a point where I feel like I need to find my own "friend" which goes so against everything I believe in and have want. It hurtst to watch your husband fall over another woman, especially one who has never had children and has no idea the heartache and nightmare we have already lived through. What kind of woman choses to try and "help" a married man when his family is in crisis and they need him more than ever? Help me please....