They are both adults, yes? How about you let them figure out what will or won't work in the manner that adults do? Yes, she may be late in learning lessons the rest of us did at an earlier age--it's still a part of growing up that she needs to accomplish. He may see quite different things in her that lead him to believe it would be good for her; whether he's mistaken or not really isn't your concern.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.