When Harry and Cedric were transported to the graveyard by the portkey, they saw Wormtail coming to them carrying something. Harry's forehead starting aching wildly so he was unable to duel but Cedric was up and could have fought. But it seems from the passage that it was a complete surrender.
From far away, above his head, he heard a high, cold voice say, "Kill the spare."
A swishing noise and a second voice, which screeched the words to the night: "Avada Kedavra!"
A blast of green light blazed through Harry's eyelids, and he heard something heavy fall to the ground beside him; the pain in his scar reached such a pitch that he retched, and then it diminished; terrified of what he was about to see, he opened his stinging eyes.
Cedric was lying spread-eagled on the ground beside him. He was dead.
My question is, Cedric didn't know that there was Voldemort in the rags Wormtail was carrying. He was just attacked by a stranger and he should have fought back. But he didn't. I know he was still a student and Wormtail was a fully grown wizard but few minutes later his junior in school was dueling with the most feared dark wizard. Cedric wasn't a coward for sure. So why didn't he duel?