It has absolutely nothing to do with you. His child is trans, and he's trying to help her access care which would pause puberty, giving her time to grow up and decide if she wants to proceed with transition.
I wouldn't condone him doing it without medical supervision, because not every trans kid is suitable. They aren't handed out like Smarties at gender clinics.
Puberty blockers have been used for decades on cis children experiencing precocious puberty - and they're still being given now. It's a proven medication.
Leave the poor family alone, and maybe reflect on the damage done to trans children by anti-trans activism. This dad is trying to save his child's life because she's been denied even the basics of care here. Trans kids don't even get proper counselling despite being in deep distress and suffering horrendous bullying.
There are kids dying all over the world, why not try saving them instead? This one has a parent who loves them. Even if he doesn't succeed, that child has a massively increased chance of making it to adulthood because she knows her dad loves her.