The F/O did all he could reasonably have been expected to do in this instance, by expressing his concern with the continued approach and landing, backed up by the aural warnings generated by the aircraft systems. That the Captain ignored both the F/O's concerns,
The F/O failed to take direct action to prevent the accident. Words mean nothing apart from covering himself legally via the CVR. The obvious action is to order the captain to go-around and at the same time reach over and select the landing gear lever to up. The F/O had ample opportunity to do just that. Assuming the captain was not insane then this action would have forced a go around as no pilot in his right mind would then press on regardless and deliberately land wheels up to make a point.