There are two ways you can deal with this but at the end of the day you are going to feel the hurt and the trust over time can or can't be gained back. I'm not saying what the best friend or the husband did was right by any means but shit happens when you least expect it. If this is from the best friends email it does read like she is sorry but her heart is saying otherwise and who knows what else.

The 1st way to deal is going with your initial reaction which is to freak the fuck out and like everybody here seems to be saying is that you need to kick ass. Yes that will be instantly gratifying but it doesn't solve anything. The hurt and bitterness doesn't leave and they will try and justify blah blah blah. If you love your husband part of the union is yes staying faithful but you are now one. Mr. and Mrs. Are your finances joined now? Have you bought a house? You gotta think about shit like that. I know your not just with him for the sexual monogamy of the relationship but for the fact that he has been there for you for 4 years and you have gone through so much together and have a bond. If you feel you can't ever trust him or look pass this then let him go and divorce him but the manner in which you confront them can make all the difference.

The 2nd way would be stepping away long enough from your emotions and confront calmly and rationally. You are kind of lucky that you have her point of view. You would be surprised how from a mature confrontation they would feel even more like shit b/c you aren't responding in the typical manner. Everything is in your court for how you plan to deal with this but you got to understand you are in a marriage now not a boyfriend-girlfriend shindig. For this way of thinking it takes a lot of SERIOUS restraint on your behalf. You never know this could be a real test for you.

When I first read the "hypothetical situation" I was like wow that's fucked up and it would suck to be in that kind of situation where you know you are wrong, know that you have hurt two people that you care about, and even worse you still want what you know is wrong. I still believe it is fucked but shit does happen to good people (if you even believe they are good people anymore). Temptation is real but then again so are the consequences.

Do what you think is best and good luck.