Hello, I'd like to help you with your question.
Your daughter in law has experienced a very painful trauma with her husband's affair with the maid of honor in this wedding. Although she may have abandonment issues from her past, the fact is that anyone would feel as she does about their husband straying with another woman. She is probably shocked, upset and sad about the loss of her trust in her husband and possibly her marriage.Add
to that the bride is offering support to your daughter in law but then going behind her back and saying bad things about her. So your daughter in law is being betrayed by the bride as well. As a result, your daughter in law is probably feeling overwhelmed and alone.
Your daughter in law is doing the right thing by seeking therapy. She needs support right now and also needs to decide how to handle her marriage. It is wonderful that you are being so supportive of her and of your son, trying to help them both.
With what your daughter in law has been through with the bride and your son, it is advisable that she does bow out of her duties as bridesmaid. If she tries to force herself to face all of these people, it will only hurt her more. She will have to participate in activities that involves three people who have hurt her deeply. And nothing good can come out of it. She is only exposing herself to more pain by going through with it.
To deal with the rumors about her from the bride, try to enlist as many people as possible to counter the rumors when they hear them. So if you go to a family function and someone mentions what was said by the bride about your daughter in law, you can say that there is more to the story that is being left out and it is best to let the bride handle her feelings with your daughter in law directly. The more you stay neutral and try to minimize the rumors, the faster they will go away. Ask other people who understand the real situation to do the same when they hear others talk about your daughter in law.
Keep encouraging your son to seek help. He is going down a destructive path right now and may not see the damage he is causing. Ask others to talk with him, preferably his father or other male relatives who are supportive. Eventually, your son will probably see that what he is doing is hurtful. It may just take time.
I hope this has helped you,