DC's biological father has no contact with DC (his choice not mine) but his gf has decided she wants to have contact when he's not there. He won't allow contact if he's present as that would mean he has to have contact, which he does not want, so arranging proper evening/weekend contact is not possible.
DC(14) has skipped school and gone to her house and she has phoned in sick for DC, she has bought DC vapes and WKD, allowed DC and DC's friends round her house during the school day whilst DC is bunking and has encouraged DC to lie and hide their relationship, hide DCs discord account from me etc. I think this is out of wanting to be cool step mum rather than anything more sinister but DC has serious MH issues and SEN so is more vulnerable than most 14 year olds and sees things as very black and white, so if its OK to lie to me about that, bunk off there etc DC thinks it's OK to do that with other people too, which makes it hard for me to keep them safe.
I have explained this to the gf and asked her to back off but she says she will continue to allow DC to her house as and when she sees fit and that it's not my business who DC talks to and when.
Is there anything I can do?