Just my two cents: I don't think you should tell the mom anything. She knows why her daughter is avoiding her, even if she won't admit it. If SD doesn't want to speak with her mom, that should be respected by all concerned. Of course, the mom won't play by the rules, but you and your husband can shield your SD from her mom simply by not asking SD if she want to talk to mom when she calls. Just keep repeating to the Mom that SD does not wish to speak with her. Be consistent.
Of course SD wanting her mom not to be alcoholic is unrealistic, but it's still what she wants. I'd stop asking her about whether or not she's heard from Mom and just leave it be.