Dear Clueless, I can understand your reaction with your mother calling you in tears, sure your AS is going to do her life irreparable damage and passing on all the news from her BF. Your AS does have a lot to lose, but no matter what you or her mom or BF do, she's the one who is going to decide if they're worth it.
You or anyone rushing in to fix things is almost certainly the wrong move. If she doesn't see she needs help yet she will push back, because that's our natural reaction if we're being shoved a direction we don't want to go.
If she ever decides she wants help, and makes that decision herself, she'll be much more likely to succeed. If the worst case scenario occurs, loss of job and BF, maybe health consequences, she might get serious about stopping the damage. So far she hasn't seen real harm to her life.
Possibly your best role in this is to calm your mother down. You mention she's a life-long alcoholic so part of her panic may be that she's blaming herself (as an example) or she see's herself in your AS's life. If your AM was thinking logically, her own recovery would be the best way of helping the situation.
Resist any wish to rescue your AS. You may be able to do something in the future, so telling her you're there for her is a great move. Think about her staying with you if she isn't sober as she'll likely be drinking a lot either openly or secretly.
Al-anon (for the F&F rather than the As) would be a great resource for you and your mother.