Perhaps you could direct her husband to resources that might help him.
The Friends and Family subforum on here for example.
Nar-Anon groups and literature
You have made it clear to him that your family are happy to support him and his daughter. This might be important if your old bestie has a stint in rehab or if he needs extra child care favours to enable him to get to meetings and find the support he needs.
If your freinds is actually using in front of her daughter this is presuamably pretty far down the line - or her denial is such that she feels like nothing can touch her.
As a close family friend and experiencing the frustration of powerlessness and worry about this, you too might benefit from attending some Nar Anon meetings and listening to the eperience strength and hope shared by others who have walked this path ahead of you, and who are more likely to to know about local resources.
Do you think your freind would be likely to discuss this with you if you broached the subject in such a way that you were being caring and supportive, and asking if there is anything you can do to help her break out of this cycle? After all, it wasn't her husband who told you. Her little girl (in all innocence) mentioned what she had seen to you, and you put two and two together. Actually, it is only her who can make the decision to get clean, but knowing that a close friend knows, still loves her and is there if she wants to get out of it might be the thing that helps her to realise that recovery is a real possibility. She may not even realise that her daughter saw her - the shame of this might be a motivating force.
Prayers going out for your friend and her family and the others affected by this situation. It is very sad.
BB