You can't fix stupid. I have friends like this and I've tried to explain to them (using small words, talking slowly, and drawing pictures with crayons) that someone else being trans isn't their business and has nothing to do with them but there's just so far I can go without outing myself to them so I stop at a certain point. Too many people feel they have an obligation to moralize and judge others, the world is full of stupid, ignorant people.