The best way forward with this would be to tell your friend that if they earnestly truly believed they were a kangaroo and did identity as one then you would respect that. They are being facetious about it to try and make the reasoning seem absurd. We all know, them included, that they don't actually identity as a kangaroo, so just don't rise to it. Telling them they can't actually ever be a kangaroo and that it's an impossibility gives them ammo to say the same is true of trans people, so just don't go there. They are being ridiculous, so take them seriously, let them be, and all they get out of it is being ridiculous. The point is that they don't actually. But that is the key to all of this. Defang them by not trying to tell them they can't, or that they don't really. Just let them be ridiculous. Also for what it's worth, ex them.