So, he made her think she’s a he?
Puberty is not fun.
So you take a girl who is on the edge and tell her that if her changing body makes her uncomfortable (of course it does) then she is really a boy in a girls body and if you take hormones you will feel better.
The bad part is that you do... for a time. So you think that the experts, who your parents told you to listen to, obey and trust, must be right. By the time the confused young person realizes that the experts were wrong and what she has done is now actually making things worse the damage is done.
And it is often irreversible.
Her body will never be normal again.
She might be sterile. She may never enjoy sex. She may have cancers.
It really has a lot in common with drug pushers.
A good way to combat it is first, don't put your kids in public schools, the second is to teach your child biology, humans are mammals, there is no such thing as a "transgender" or hermaphrodite mammal. The third is to teach them about puberty, yes it is a change and it is usually not fun, but your body settles down and things do work out.
Of course all of this means that you will need to be involved in your child's life and acting as their parent. It helps if you have a spouse who is willing to be their parent as well rather then just a biological contributor. So pick your spouse carefully.