I'm an alcoholic and have been sober now for nearly 9 years, so I hope my perspective may give you some insight. I’ve NC’d for this, for obvious reasons.
I know that at least one of my friends had become aware that I was drinking a lot and at times which were not appropriate and I now know that she discussed these concerns with my DM and DSis, who were also both already worried themselves. My DH was blinkered to how bad things had become and was dealing with a lot of work related stress at the time. Had my friend tried to talk to me about my drinking, I'm sure I would have been defensive, would have denied it was anywhere near as bad as she thought and would have rejected any offer of help.
As it was, my DM stepped in - prompted by my friend having called her. My DM sat me down and told me that if I carried on drinking as I was, she would report me to SS herself, would have my DC live with her and I would be dead within a year or two and my children would barely remember me.
I made all the right noises, promised to stop, but still kept telling myself "it's fine, everyone drinks, it's normal". My drinking behaviours became sneakier and I struggled to manage 'life' - I had to start walking the kids to school as I was too drunk/hungover to drive, so had to lie about why I hadn't taken the car, I even snuck out from the office to drink vodka in a park one lunchtime. I was out of control.
Within a month my DM turned up again, I arrived home to find that she and DH had spent the afternoon having a heart to heart, and the next day I spoke to my boss and arranged a 3 month sabbatical, and my DC and I moved in with my DM, 150 miles away.
I registered with a local alcohol charity and found details for AA groups, Mutual Aid Partnership Meetings, Women's Addiction groups and 1-2-1 Counselling. I attended a group or a meeting each week day and my DM bought a breathalyser and tested me twice a day. I knew that this was my last chance, and if I blew it, I would be moving home without my children - and would probably lose my DH too.
Your friend isn't going to stop drinking until she's a) ready, or b) more scared of losing what she'll lose if she continues to drink, than of losing the alcohol itself.
Is your friend close to her own DM and is she someone you feel you could approach about this? If so, I would do so - and would say what you've said here; that if you've noticed you're concerned that others (school, SS) will also become aware and you want to offer her help before irreparable damage is done. Hopefully her DM would be able to broach the subject with your friend's DH, then the 3 of you can stage an intervention - an unexpected (by the subject) meeting of concerned parties to raise an issue which the subject is trying to avoid, with the aim of getting them to face up to something. You will need to have a list of contact numbers for Alcohol and Addiction groups, some websites she can use and ask her DH to make her a GP appointment for the day after the intervention, so she has to follow things up immediately. You will need to lay it out to her, exactly what she is at risk of losing (everything).
I can't promise you that this will work, but it worked for me and I have always been the biggest drinker I know. In the beginning, I felt as if without alcohol, nothing would ever be fun, ever again (how sad is that?!) That's why in AA they say "one day at a time". I love my life now - DH and I have been together for 20 years and having been through my alcoholism and recovery, I know we can deal with anything now. Our DC don't have any memories of "Drunk Mum" (for which I am very thankful), but they are aware of my addiction history and the genetic predisposition they now have (I have since learned that both of my Grandmothers were alcoholics, as is my biological DF). Our DC appreciate that if their Dad has a few pints after football, then in an emergency they will always have a sober DM to deal with things - and I like being that person, the one who can be relied on to be sober, and I never thought I'd be saying that!
Whatever you decide to do, I feel sure you need to talk to someone involved about it, as I know that if something happened and I hadn't, I would feel shitty. It's incredibly hard to know what to do for the best without knowing those involved, but I wish you, your friend and her family all the best.