Here are a few resources that might be shown:
https://www.susans.org/forums/index.php/topic,197523.msg1756901.html#msg1756901Especially the website from the unicorns dad ... its written by a parent showing some restraints cis people might have ... is it a phase (its likely its not) ... did we cause this ... (no people are this way) ... etc...
There are studies showing there is a biological connection to being transgender, to do with brain development before birth.
There are differences in brains of women and men, fixed by various transmitter substances.
So its how people really feel, and its nobodys fault.
The usual remedy is to adapt the body to how people feel its necessary.
There are many happy transgender people.
In fact many people experience a kind of relief once they start hrt.
*hugs*