Your choice of doing something about it, or not doing something about it has nothing to do with their codependency, that is the type of relationship between them.

I'm sorry you are worry about them, I would be too. Perhaps this is an issue of recognizing everybody involved is a "ground up" and have free will to decide what they want for themselves. The three of them are as OK as they are going to be with the choices they made.

When the situation changes, they'll need to make new choices. Just make sure to be there for them when this happens. Other than that, they have pretty much told you to leave it alone, right?

gab