The thing about JKR stuff is that he might not fully understand the transphobia involved. Is this an isolated incident, or is he not supporting your friend in other ways?

We're at a moment of reaction to trans rights, where opponents of LGBT people in general are looking for whatever hooks they can get to wind stuff back. That includes pretending that JKR was attacked out of nowhere, a sort of spiritual "definition of a woman" thing that is ultimately very conservative, and this targeting children thing that twists helping kids with abusive parents into something insidious. Your friend's dad is falling for that, because it's a specific message targeted at twisting his morals this way. All you can hope is that he loves his kid more than whatever this is.