Do I do something or not get involved?

[Firefly76]

Last night my daughter's friend came over in a bit of a state after an argument with her step mum via texts.

She trusts me and feels comfortable talking to me about it. But during the conversation she disclosed that she steals stuff. Hundreds of pounds worth of stuff from shops. She's 13 nearly 14 and has been doing it for 3 years now. She has a whole system/technique on how to do it. I asked if she was addicted to the rush of doing it and she agreed. She admits it's wrong but she can't help it, and she doesn't want anyone to know, especially her mum who she lives with.
This child has a lot of problems at home and at school, so didn't have anyone else to talk to. I feel she needs someone understanding and supportive in her life, so I want to keep that door open for her, but I also feel I should tell someone so she can get help before she ends up in jail. But if I do that she will no longer trust me, and she'll have no one. I'm also obviously concerned about letting my daughter go shopping with her friend. My daughter says she would never steal, but if her friend did and they got caught, my daughter would get in trouble too. But the friend says my daughter helps her with the urge to steal, via texts, (my daughter hasn't gone shopping without me yet).
I don't know what to do. Any advice would be gratefully received.

well for a start
at 13/14 not a chance that my daughter would go out shopping with this girl ever again

and what i would do…. is have a chat with the school about how concerned you are about her family situation

This. It sounds like this friend has absolutely no support other than your DD, perhaps relying on her too much. That's not healthy for either of them.

I'd reach out to the school & share your concerns. Also advise your DD not to go shopping with this girl, there's plenty they can do without going into shops, she doesn't have to end the friendship or distance herself if she still wants to be friends, but putting boundaries in place to protect herself from legal troubles and also show her friend that she's not comfortable and disagrees with her actions is a good idea, it might even get the other girl to stop if she doesn't have your DD there as support.

your daughter is never unsupervised with this child ever again. Shop lifting wont be the only rule breaking going on, and your child will be heavily implicated.

She wont go to jail for shop lifting, She might get help if she is caught.

Tell the school anonymously - or ring NSPCC for advice

Make sure your daughter knows never to accept a caution.
Obviously don't let her go shopping with this girl but things happen and if they do she needs to understand where she stands.

They do not go to the same school, so I would be contacting a school I don't know. Would that be weird?

when I was 14 I had a situation where my friends were shoplifting and I was with them at the time and unfortunately ended up with a reprimand just for being with them . It was a horrible horrible experience and I would hate for your daughter to go through the same. Please don’t let her go shopping with her, like others have said nothing wrong with socialising at your house etc instead. Would it be possible to speak to the school with the girls consent? Is there a possibility there’s a trusted adult at the school she could also speak to.

no

and quite frankly who cares if “weird” if it is patently the right thing to do

i would be bloody concerned and cross that my daughter was being used to stop someone committing a crime

they don’t go to school so i’d talk to the girl’s school and i’d limit contact between my daughter and this girl

my daughter is my priority and if that means distancing her from this girl, so be it

You can contact her school. It would come under safeguarding. You don't have to have any link with a school to report a safeguarding issue.
Do it quickly as most schools close in a week or so time.