My two, by my first marriage, were somewhat older than the Brat when their mother and I divorced but, although they both cheerfully accepted that I had, whilst separated from their mother, acquired an girlfriend and ultimately married her they were horrified that 'Mum' should have any male friends, go on dates etc. One of the reasons that my daughter was sent to live with us was that her mother complained constantly that the child (then 16) was interfering with her love life and rejected totally any male friends she had.
My son (then 18) summed it up in thus "It's OK for dad to have a new lover BUT Mum's just can't! It's not right"
It seemed that both children thought this and that our daughter made sure that her feelings were known to all by her behaviour ... although she never put it into words and still won't admit to it despite the fact she is now coming up to her 41st birthday and has herself divorced her daughter's father and eventually, after a number of boyfriends, remarried.
I suspect that a part of her/your problem is teenage angst and part as above. A very toxic mix as I discovered.
