You sound like an equally heinous friend. Your friend has YOUR back before she has anyone else's back. Her cheating on her husband does not affect the friend's life. Now the friend sending her own friend going down tailspin of overeating and drinking? Sure. But that man AND woman married each other, not the friend. This is absolutely without a shadow of a doubt a decision she made that only affects "the involved parties," and if you somehow think your opinion of your friend's marriage matters more than those two people, you sound single and miserable. (And I'm saying that as someone who is not married.) I don't "hate" anyone but the Klan and white supremacists. This "terrible" thing you think she did doesn't have zip zilch to do with paying your bills, getting your groceries, funding your family or paying your mortgage/rent. MIND YOUR BUSINESS! When the vows are recited, they don't include each other's friends, regardless of what your imagination is telling you.
If you "hate" someone for an indiscretion in their relationship, then you are a busybody with no life who spends most of your days judging everyone else and probably doesn't have a thing going on in your own world. Work on yourself. Don't be a Nina. And if you are, I hope you don't have any friends who have to deal with this insufferable attitude.