It irks me that you would rebut this guy with this argument simply because I know that the empiricism strongly leans toward personalities remaining very stable throughout the lifespan, and that changes move together as a population than within-individual. Academics might not have yet built consensus on a general personality theory (Big 5 is more a descriptive research framework than a theory), but there is consensus on personality empiricism.

Surely there is another angle of counter. I don't believe in winning argumentation by any means, especially by scientific disinformation without empirical cause.