How do you distance teenagers from toxic friends?

[WWYD2016]

DS is 15 and in his own words is having the best school year ever after years of drifting in and out of friendship groups.
His current group of friends are all female, he finds the boys immature.
His friend suffers poor mental health after years of neglect and was placed in foster care at the start of the year.
In January I found weed in DS possesion, he was grounded and I went through his phone and found many texts from this girl demanding my son's presence by 'crying wolf' using excuses of being in danger of harming herself. She texts in capitals urging him to hurry.
DH and I have tried to explain to him this is manipulation and though its good to be a supportive friend he cannot fix her.
Whilst grounded one day he didn't come home, he couldn't, friend was upset after a colon cancer diagnosis, its a lie. She apparently has thyroid issues, it too is a lie. Last night he missed a school concert taking her to a walk in centre because she was ill...she hadn't eaten or drunk for 48 hours.
I collected my son yesterday evening from her carers and was invited in for a 'chat'. Carer said if DS was her child she would not want him friends with this girl. She is manipulative and is afraid of losing her friends so threatens them with her health and safety. Apparently at their school the staff are worried about her toxic group of friends, poor kids all have issues of some description and are creating a negative breeding ground amoungst them.
My son has a stable background and is well cared for, he finds these girls exciting, not only are they attractive, articulate and intelligent but they're feral and wild.
I have talked and talked about this with him, friendships are built on trust and love not manipulation and threats but I am a mom, what do I know.
What can I do to seperate them without leaving them more desirable and DS lonely?

Have you spoken to the school?

Oh dear, it's a very difficult problem. You have to just hope that your ds will eventually see this for himself, because at that age the more you go on at them the more they'll dig their feet in the ground, ignore you, or worse. get deeper in.

Something similar happened with our ds ; he got through it (he eventually saw his strange friends for what they were, but it took a while and he listened to his sister not us!)

I am in a similar situation only they are boyfriend and girlfriend, both 15 she has done the same lied about a terminal illness, threatening self harm etc. I did contact the school and spoke to the child protection officer and their form tutor, who to be fair had identified it was a toxic relationship. I also confronted the girl and told her I would not allow her to control and abuse my son but would help her get support. She's having counselling now but I keep a very close eye on their relationship and contact outside of school. I won't stop them from seeing each other, he needs to want to do that not me take the control away. We have talked about what healthy relationships are but I would ideally prefer them to be apart and pray that at 16 she doesn't become pregnant!

At that age my dd had a horrible group of friends and ended up addicted to weed and upshot was she failed her gcses. If I had the time again I still wouldn't have forbade her from seeing them as she would've seen them anyway. Letting her decide for herself that they were poison meant she hung with them for 3-4 months, I reckon it would've been much longer if I'd have interfered. The aftermath was awful mind.

You did right fessmess She could have failed her GCSES without the weird friends and drugs, our middle dd did!

She went on to college though when she old enough to see the benefit!

JiltedJohnsJulie yes I have, they've promised to keep an eye on him. I was utterly shocked yesterday evening when her carer told me school had called HER because MY son had had an emotional breakdown before half term because he believed his friend had cancer. Needless to say I am awaiting a call from his Head of Year, cannot believe I was not informed.
I have tried gently to talk to him both last night and today, I am not banning him from seeing her but I want him to tell her it is not acceptable for her to lie to him, it is not the basis of a true friendship, unfortunately he won't he does not want to hurt her because she is fragile.

It's not easy but one can't have one's life ruled by 'fragile' needy, disturbed people (there are plenty of those people in all walks of life and at all stages of our lives).

Your son needs to be able to find out how to cope with them , it's a process of growing up, it takes time….. ahh bless him.