Here's my .02... anything two (or three, or four..) consenting adults want to do that doesn't hurt anyone is their own business. To the pursuit of happiness! If OP's friend (co-worker? I forget) and her husband are content with their arrangement, good for them! More people to drive the kid to soccer practice!
That being said, my husband and I have some very good friends who are swingers. We met them in line at Universal and they are the nicest, sweetest couple you'd ever meet in your entire life and have probably the best, healthiest marriage I've ever seen. They've been very happily married for a little over 10 years now and just welcomed their first child into the world a few months ago. They don't go out to date or form relationships independently of each other, but do meet others to spice up their marriage.
Recently, they had a BBQ at their house that we were invited to. After a while, we found out that some of their other friends there were also swingers. We all got to talking a bit later in the evening, and once they realized that we weren't going to grab a torch and chase them out of the village, were seriously the nicest, most welcoming people we've ever met. Some were DINKS, some had children at home, and some had children and grandchildren. They had all been married for 15, 20, 30+ years and couldn't be happier. They all had very normal jobs and very normal lives, with very normal children. Some of their adult children knew, some would never dream of telling them. I can't get over what a relaxed atmosphere it was- everyone was so non-judgmental and friendly.
So... I wouldn't generalize by saying ALL open marriages end badly or are bad to begin with. If a marriage is bad, it's going to (or should) end whether they decide to have an open marriage, or if one partner decides to cheat, or if one partner doesn't do the dishes on time.