I agree with what everyone else posted.
I would like to add one thing, though, as this logic may also help the guilty feeling.
Originally Posted by
marcia36 
I feel relieved, thinking this friend may get help instead of having to hit bottom, but I also feel like a tattletale and two faced.
Whatever she'd feel due to the consequences of this phone call pales in comparison to the grief that she'd feel if she were in an accident, and one or all of her children were injured or killed. You're helping to protect her children from that possibility. I am sure, if they knew, they'd be grateful to you for your courage to do the difficult thing.
You did the right thing.