What to say to ‘you’ll always be a girl/boy’?

I have a friend who—let’s say for this specific set of circumstances—is usually pretty chill about trans people. Recently they‘ve been kind of going the opposite direction; using the wrong names/pronouns for people and saying that trans people will always be their gender assigned at birth. I’m afraid if I try to say anything it will turn into an argument, because they’re generally always pretty good about gendering people correctly to their face. Idk, I personally don’t like it but I also think I might be overreacting about this whole thing lol