Agree 100% with Slevin. As a rule of thumb, I try to be very careful with the "eye for an eye" logic. I've had it work out not so well for me in the past with much more serious things. Plus, if the husband really IS an ass, this one could quickly elevate to that "pretty serious" level.
One of Mrs. Mix's playmates was married (bad marriage, lousy wife, yadda yadda, whatever) It's the drama of that whole situation that really lead us to abandoning the open marriage.
Do what you want, but at least don't delude yourself into thinking that this situation won't lead to problems. It doesn't take intimate knowledge of all participants to know that. As a matter of fact, LACK of intimate knowledge of all participants probably makes it easier to recognize.