“...how can a U.K. citizen fighting for Russia be declared a traitor?”
Don’t know if he can be, but remember that the British definition of ‘treason’ might be quite different than the U.S. Constitutionally-defined definition. In fact, the Founders defined treason in the Constitution because the charge was so abused in Britain.
"Don’t know if he can be, but remember that the British definition of ‘treason’"
True. To Henry VIII, treason was making up slander about your wife in order to get rid of her. Basically, anyone who pissed Henry off was labeled a traitor, and guilty of treason.