The first thing they need to realize is that they aren't really to blame. What can start out as something to do just for fun can turn into a nightmare that's beyond your control. I've known many people who have been addicted to many different drugs and it always starts that way. The drug takes control. They need to realize this and not beat themselves up over it.
What they need to do the most at this point is four-fold. One, they need to stop blaming each other and start supporting each other in their recovery. Two, they need to stay completely away from the drug scene. If they don't their recovery will fail. It's just that simple. It won't be easy. It will mean having to drop the entire circle of friends and aquaintences who are also into the scene. Three, they need to remember what brought them together in the first place and try to recapture that. Four, they need to think about their children and what they mean to them and use this as a motivator to to succeed with the first three. I don't know their full situation but my guess is they can get their children back if they pull the first three off.
All this will be hard as hell but it's the only choice they have. From what I've observed over the years(I'm 51, believe me I've seen this sort of thing many times.) this is the only way that they will be able to pull things back together. I'm rooting for them.
As far as your involvment goes - support them as much as you can but don't push in any way.