I'd try to reason with him or her.. Mixed-race kids are seriously lacking when it comes to identity, having no people to call their own and serious issues with disassociation when they grow up. Such a child is better off not being born. Can a mixed-race person live a happy life? Yeah, I guess. Would they be better off as entirely one race? Absolutely.

I agree most with u/MortalSisyphus on this issue; white on castizo would produce a child with no real mixed-race issues, as, as far as racial ingroup bias goes, they're white. white mestizo would produce a castizo, which is less preferable but at least castizos have countries they can call their own. White on black would produce a mulatto, who, outside of Brazil, don't really exist in the rest of the world and honestly never should have been.