It has been said before, but I need to drop my vote in for face to face. Not confrontation, but discussion. Even if it is one sided, if you can get her to think about this you have a chance of helping. She has to be forced to accept that she could be damaging her children. A mother that can continue on a path that they know is detrimental to thier children is not a mother at all.
Ask her questions, ask her why this is so important she is risking her safety, sanity and her children to do it. Get her to think. I don’t know how you can start a tirade about what she is doing, but I do know that getting her side of it can be done with questions. A guy with legitimate questions is much harder to ignore than a letter with good, but deniable, points.
She needs to get a clear view of what is going on. A prespective can only be found when you question what you think, what you do. Make it clear that you are willing to help her through whatever choice she makes, but you need to know that she is making a choice rather than just letting everything flow.
More power to you, and her. The friends that truly help you out are most precious.