Girls rarely kill their rivals. They might fight and sputter...but kill??
I believe a girl - any human being - who is in a drug-hazed trance will do anything. I believe she had a role in the murder. That’s my opinion.
What happens if they find her guilty (again)? Does she go back to jail in Italy?
Not as often as men but it does happen.
They also typically use firearms, poison or hired help in murders due to their physical weakness.
I’d say it’s highly unlikely she attacked the girl with a knife on her own. If anything, she was helping the man.
I don’t think an accident due to weird sexual play is out of the question either. Women are typically very secretive about such things and it may be the reason she was so evasive during the initial interviews.