Edit for context:
My view is transracial isn’t valid and this person is trying to dogwhistle. I’ve already blocked this person, and now they’re going after my friend saying my friend is transphobic because they disagreed with them about transracial being a thing (they’re purposefully leaving the context out so my friend looks transphobic when what my friend really said was transgender is valid but transracial isn’t)
The correct response is to consider what the correct way to synthesize the positions is, and go with that. There’s nothing wrong with adapting your position to handle possible inconsistencies. The goal is not to win but to be the most correct.
Typically, the assumption is that this is an argument that transgender is invalid. Perhaps there’s another way of looking at it. Perhaps a way people aren’t ready for, which is why your opponent went in that direction.
Alternatively, it can be pointed out that this is changing the topic, because it technically is.