So have NC for this.
My DSD was a lovely happy person up until 13 and all of a sudden it all seemed to go off the rails.
DH was (and still is) very active in her life not a Disney dad and we had not far off 50/50 residency.
Time with us gradually went downhill - which we put down to usual teenage behaviour. We found out from her DM that she had a boyfriend (who turned out to be 16).
She was unusually tired.
We found she had been up until 3/4am online. We cut wifi after 10pm.
Then a call from the school. She'd refused to remove her numerous bracelets for PE. The reason - they were there to cover self harm scars (that we later found were also covering all her legs).
Much counselling and therapy.
Upshot is she was allowed by her DM to attend a number of sleepovers that turned out to be unsupervised (her DM did not know this and would not have allowed it if she had).
What had been a normal teen relationship got pressured into a sexual one at a very young age and she was not equipped to deal with it.
He'd end it with her, reconcile just pre- sleepover and the circle would start again. Self harm was her way of dealing with this. Thankfully he moved away and out of her life - but this was only just after we knew what was happening.
At 15 she was already on her second sexual relationship. Again - hugely intense and full on. It impacted GCSE's, friendships etc. She just didn't know what a normal relationship was.
We are thankfully many years down the line and she is really doing well.
But it's been a hard slog.
A sleepover sounds so bloody innocent, but if your child is unlucky enough to come across a manipulative older boy like DD did, the ramifications are huge.