Rotten, I know that you and your wife have been through some major devastations....disregard anyone who says stupid shit without knowing A FCUKING THING.
I can understand why she would be upset w/you for feeling (I guess loyalty might be the wrong word here, but it is a close as I can get to express what I am thinking) these "friends". I also understand feeling bound by your word. I too feel that if you don't have your word....what do you have? But there comes a time when you have to realize that keeping your word will do more harm than good. (As I feel it will in this case.)
I think this girl presented the situation to you because she knew how strong your character was and that her "secret" would be safe w/you....but her behaviour is BEYOND self-destructive. She is taking other people's lives into her hands...people that she will NEVER EVEN KNOW.
We are all human and we all make mistakes. As long as we recognize and learn from them, it is OK. You are allowed to break your word in this case, if she is not strong enough to do what is right herself and alter her behaviour. You have done nothing wrong, except perhaps try to be there for someone who needed help. If you can not convince her to step up to the plate and maybe even get some counseling, then I feel that you have fulfilled your obligation and then need to do what is right.
As for you and your wife. It is not that simple. I feel for you both - BELIEVE ME. If you guys can not find a way to work this out it may pass with time, but will cause a rift which will lay dormant but crop up and grow bigger with each difficulty that life throws at you. Don't let your relationship go down the tubes. It is damned hard to find a true friend to go through life with. I know better than anyone, I know.
Work it out. Get to a counselor, talk till you guys are sick of talking and then.....talk some more. Do it. You will not be sorry. Let it slide and you may regret it somewhere down the line.
...Just my .02