Borderline/ Alcoholism any such thing as useful intervention?

[dogdrifts]

I have a lovely friend who has recently been dx borderline and is a lifelong alcoholic. She has been in and out of residential rehab a lot in the past year. I have been contacted by some mutual friends (in a group along with the dh) who are concerned that her drinking is spiralling again and she is frequently drink driving. They are going to report this so that if the local force see the car being driven erratically, they can stop and are aware that drink driving is a high likelihood. (I have no particular concerns with this approach - if she kills someone and we haven't reported it, we are just as culpable). That said, some of the group feel she should be warned in an intervention-style manner, so that she can choose/ decide. I am concerned that this may be counter-productive to her mental health - essentially we are the only people left standing by her, and she would feel that this decision is effectively tantamount to being abandoned by everyone. She has made several suicide attempts in the last year, and is a flight risk (she disappears with litres of booze and pills periodically, having sent final messages to everyone) the pattern of her drinking and MH suggests that she is in a downward spiral again (which makes drink driving/ running/ suicide attempts more likely).
She is validating the drinking using her new borderline dx (x% of people w borderline have addiction issues, and it's 'just' alcohol, not crystal meth) and friends who have stayed with her recently say she is drinking fairly constantly throughout the day (but hiding it - she will make an excuse to go upstairs/ has bottles hidden around the house) but telling everyone that she has a glass or two a day with dinner. She thinks nothing of hopping into the car and popping into town (usually to buy more booze). We are fairly sure that her counsellor is not aware of the amount she is consuming. Her dh is exhausted, and has given her so many ultimatums about her drinking that he agrees the police are a logical next stop in order to prevent harm to an innocent party.
I feel utterly duplicitous, but am certain that telling her would be counter-productive and would precipitate another suicide attempt/ runner.
Does anyone have any wise words how to best approach this? We all want to continue to support her, but have serious concerns that sooner or later she is going to hurt someone badly, and neither she nor we will be able to recover from that.

That was really long. Sorry.

She's using the alcohol to treat her borderline. Without treating the borderline you can't take the booze away.

Prevention is better than cure, therefore she should be told that the police are aware of her drink driving.

Surely she needs to be sectioned if she is a risk to herself??

She's on about 4 different medications for her borderline, as well as something to reduce alcohol cravings. She sees the counsellor every week for borderline, and is on the waiting list for DBT.
It's not so much the choice to drink iyswim (I don't mean intervention in the 'stop drinking' sense - that's her decision) but in terms of the drink driving.
It's the drink driving that is concerning everyone. Apologies if that wasn't clear.
She is clearly finding managing her new dx very difficult, but that can't be used as an excuse for drink driving?

Quite - she has been in and out of mental health units (and associated rehab) for a year, but they won't admit her until the next time she tries to kill herself as at the moment they are trying to support her to live in the community, not in an institution. I am worried that if she is told, it will precipitate that kind of event.

One of the most destructive behaviours of BPD is self-harm/self-sabotage/risky behaviour.

Drink-driving could be considered to fall under this banner - she knows deep down she'll be caught (sooner or later) and that she will be punished - compounding her feelings of self-worth, etc.

If you can catch her when she's sober and drill into her (on some level without blame ?? ) that you understand she's struggling/hurting/wanting to hurt herself - but taking out others will just simply drive her to suicide (she probably won't care about that) - but it may drive others to suicide too.

Then pour her a drink and hide the car keys!

Thanks Hildur, that's helpful. We did discuss whether a better intervention may be to remove access to the car itself, but that's also risky - it would be removing her ability to get to counselling appointments (it is unlikely she would bother to take a cab) and would essentially be imprisoning her - she lives just out of town enough for it to be too far to walk and the rural nature of the place means it is unlikely she would go where or do anything with no access to a car. She is finding it hard enough to go out anyway. Such a fine line between support and harm prevention actually causing greater harm.