Were it me, I'd voice my concerns to Stephen and let him figure out what to do. He may have no problems with her cheating on her husband because he isn't deeply involved with her, which, if you're open isn't a problem.
If you're poly and not open, it's a problem, as redpepper explained.
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.