I actually wonder why this thread is in the sex and gender topic.
From reading Esther Ghey's statements on this subject, it doesn't seem as though she is specifically linking the issue of phone use to either Brianna's transgender identity or Brianna's murder.
It seems to me as though she is making rather generalised comments about how over-exposure to smartphones is having widespread negative effects on young people and making it more difficult for their teachers and parents to educate and parent them.
As others have pointed out, children committing horrible murders isn't a new, post-smartphone phenomenon. Look at killers such as Jon Venables and Robert Thompson, or Mary Bell, for example. They didn't kill their victims because of smartphones.
Esther Ghey also doesn't seem to be saying that use of smartphones turned those children into killers. I think there's every chance that unrestricted access to the internet contributed to them being able to plan and carry out a murder, but they might have done it anyway, even if they'd been teenagers 30 years ago.
What is interesting about Esther Ghey's comments is that she is focusing on the negative effects phone use had on Brianna. Even if she was totally happy with her child being transgender, which I don't think she has ever confirmed or denied, she talks about Brianna's smartphone addiction and how this most likely contributed to negative behaviour both at school and at home, and difficulties making friends with other children. The reason why I think this is interesting is that it almost comes close to victim blaming. She isn't saying that if Brianna's killers hadn't had access to the internet then they never would have killed Brianna. She appears to be suggesting that spending too much time on the internet made Brianna an unhappy, anxious child who wasn't doing well at school and found it difficult to make real life friends, all of which could make a child vulnerable.
As I understand it, Brianna went to the park to meet the killers, believing they were going to take drugs. That sort of thing could just as easily have happened in the 1990s or 1980s. It's very difficult to see a direct link to smartphone use here.
What I would say is that the kind of kids who are doing well at school, getting good grades, have lots of nice rule-abiding friends, no mental health issues and see a bright future ahead of them, are unlikely to strike up a friendship with a girl who has been expelled from another school and go to the park with her to take drugs. That is the kind of thing only troubled, unhappy, lonely, vulnerable children do. And Esther Ghey seems to think that excessive phone use contributed to Brianna being that sort of child.