He was hanged under the provisions of the 1351 Treason Act which made it treason to anyone who ‘soit aherdant as enemys nostre Seigneur le Roi en le Roialme, donant a eux eid ou confort en son Roialme ou par aillours’. I don’t think there was any doubt he was giving aid and comfort to the enemies of the king and the fact that the act says ‘in his (the king’s) realm or elsewhere’ means the fact that he was in Germany didn’t matter.