It can be helpful to point out that trans experiences are different and trying to force trans into their desired category actually erases trans experience. It's a tricksy one but has worked in the past. It also helps keep trans men in focus, who u do genuinely think are ignored as they are biologically women.

Old school transsexuals where also taught that the changing of secondary sex characteristics did not make them their desired sex, so what has changed? Ask a few questions about the difference between sex and gender.

Also rather than repeatedly adding to the destabilising of what it means to be a woman, focus on what it means to be a man and exactly at what point do they stop being a man? Usually some bollocks about 'in the wrong body' is then said and it's quite fun to then ask if they believe in gendered souls...

Essentially keep asking questions that destabilise and seek 'clarity'. If there not too far gone the questioning usually gets them thinking.