First I have to say to Darren that I have never heard, "the cat among the pigeons."
I think you need to do your venting with us or like Darren says get out and exercise, go for a nice long walk. But, you have to stay out of the middle of it. I know it is so very hard for a parent to do. But, if the relationship isn't the right one for your son, the more you stay involved, the longer he will stay involved. It just somehow works that way. My youngest daughter was in a relationship with a guy for several years who was abusive. I had to find this out from her friends. My daughter wouldn't tell me, yet she stayed with him. It didn't happen often, but hitting my daughter at all or emotionally abusing my daughter was too much for me. Believe me, I wanted to kill him.
His mother and I did go to the same hair salon at that time and she was the post master and thought she was something special. My daughter also used my hairdresser and my hairdresser had seen them out in public together and how the boyfriend acted. So, I just always made a point of talking to my hairdresser so everyone else could hear about how Dee was mistreating Tracie and that I didn't know what to do about it. Of course in no time his mother heard about it and was very upset with me for spreading lies about her son. I think she was thinking along the lines of a lawsuit until she talked to her son and a few of his friends. She never called me. Before long, their relationship began to unravel and she met her now husband.
But, I can tell you trying to pretend to be nice to him was the hardest thing I ever had to do when I really wanted to hit him over the head with something. I didn't want to alienate my daughter when I knew she needed me most.
Try to stay cool and calm.