Cheating is cheating. Swinging is mutual aggreement between everyone involved and nothing is done behind someones back. I fully understand the guy. There is a huge differrence to "fuck around" like a couple and have fun with other similar thinking couples, versus doing something behind your partners back. IMO the woman acted as horrible as in any other non-swinging relationship she would.
You can talk all about how stupid swinging and open relationships are and how they're almost always destined for disaster, and I won't disagree one bit, but relationships only work if everyone is on the same page. That goes for everything from the small stuff to the big decisions. It doesn't matter what silly nonsense they agreed to do beforehand, the fact is they agreed to do it together.