IMO, it doesn't matter who was a friend first. The fact is, she's married, and although the general moral fiber of many people nowadays has degraded, I'm still of the feeling that they made vows, so stick to them. If she wants marriage and all that goes with it (good and bad), then she should stick with it. Otherwise, separate & get a divorce. Screwing a married woman may seem cool and OK to some of you, but if you do that, i sure hope the same thing happens in reverse to you some day, so you can be on the receiving end and see if you still hold the same opinion you do now.
Separation, or at least talking about separation, is a good way also to let your spouse know that you're serious about your unhappiness with the marriage.
As far as their sex... how often they do it is between those 2. And honestly, from what you said, they were only at it once per month before they got married, so she had that info up front... but still said the vows.
htw