Take it from the daughters point of veiw (that is me) who is now blessed to be sober.
You are under no obligation to loan her the money for the car just cause you said you would. Did she say she would stay clean? did she hold up her end of the deal? I say this all time, and in saying in no way am I blaming my mom for my using/drinking. But when I was finally cut off with help from her....I found my way to sobriety faster. When she finally cut me off and I was living in my car, until I traded that for drugs.
Enabling is enabling. By you not giving her money for a car will it help her get clean sooner, I don't know, but it will definately help her from not drinking and driving.
God Bless.