"He is the only one who changed shirts."
Translation:C'est le seul qui ait changé de chemise.
It's idiomatic. In English, when you change clothes, the piece of clothing goes plural (change shirts/hats/pants) because you remove one and wear another. In French, it goes singular (unless certain case), because at the end, you wear only one piece of clothing (changer de chemise/chapeau/pantalon, but changer de chaussures/gants because you change a pair)