Disagree again.
I had three kids when I married my husband. My husband was and is not their father they have a father.
Two of my kids were ok with husband. One wasn't and was totally horrible to him. I did stick up for my son when my husband tried an angry confrontation. I told him in private to let me and my ex do the disciplining. Husband was good with that and things got better, but things have never been lovely between those too and I am fine with it. My son is mature enough now not to interfere in our marriage and as long as he knows I love him, his mother, he is indifferent to how husband feels about him.
The fact is, the mother needs to take control of a minor child. If an adult child gets into drugs or criminal activity, it is reasonable to think that a step father, who did not raise him, may resent him. At one time husband was the adult. Now they both are adults.
Every child is different. Every parent is different. Not all men can talk it out or deliver a message gracefully. My husband is the most kind hearted man I ever met, but he is not a talker about things. It doesn't work for him. We don't know is WSMs husband is the type who is able to talk out his feelings either. I haven taken my hub to therapy with me a few times and he is a good listener. But he does not talk much.
I don't think everyone can solve things by going over them all and talking. It's good if you can, but if people are not good communicators, it won't work. I'm not all that hot a communicator either.
WSM needs to solve this in a way that works for the personality of her and her husband. We don't know the dynamics. I don't feel the grown kid should be a priority here. Sorry, he has messed up. But if WSM can afford to bring her grown son and either talks her husband into being ok with it or puts son in front of husband, then he will go.
We are all over the place with this, and our own personalities and valuesv(including myself) are coming into this...old hurts from our past too maybe. We are probably driving poor WSM batty with all our various opinions...lol.
I hope, WSM, you can come to an decision that works for you and yours. You have read maybe too many conflicting ideas. Take what you need and leave the rest and good luck. 
