Not only did he return to try to rescue it, he also actually persuaded his friend Mavovo to accompany him.

They got back to the boat and began to lift the plant out when the Pongo fell upon them, striking at them with their spears over the width of our canoe.

Mavovo struck back with the weapon he had taken from the Pongo sentry at the cave mouth, and killed or wounded one of them.

Then some one hurled a ballast stone at him which caught him on the side of the head and knocked him down into the water, whence he rose and reeled back, almost senseless, till some of our people got hold of him and dragged him to the shore.
So Stephen was left alone, dragging at the great orchid, till a Pongo reaching over the canoe drove a spear through his shoulder.

He let go of the orchid because he must and tried to retreat.