You make a great point. His family will likely keep him around and give my W hell. Up my MIL did this same thing with one of OM uncles and got pregnant with my SIL as a result. This is huge reason why my SIL has spent no time with this side if her family until last summer. That family welcomed my W regardless on how they feel about my MIL. And 4 months into it she is having an A with one of their men. So yes this family will automatically view my as being a chip the old MIL block. And OM like so many times in this process when his W found out they were in contact will pull away from W quick.
So I am well aware on how this process will go. But your plan to let W fall on her face sounds good as well. The only problem is how many people get hurt before W falls on her face? And that is the thing that worries me about letting bad things happen. My job involves me preventing bad things from happening to people. So my fix it mentality is all over the place. Especially when I have witnessed dirt hand what this type of behavior does to families. And the question always asked is why didn't anyone do anything about it?