Piggy helps the boys to see reality at different points in the chapter. During the meeting at the beginning of the chapter, after Ralph gives him the opportunity to speak, Piggy explains to the boys that they have very little chance of rescue because they had flown so far off course no one will have any idea where they are.
Once the boys see that their gigantic fire has fallen in on itself, Piggy also reminds them that they could never bring enough fuel to sustain a fire like that and he derides them for being silly.
Piggy then is the one who first realizes that the little boy with the mulberry colored birthmark isn't around. Though the other boys, particularly Ralph, try to suggest that he may have gone to the beach, it is clear that he has likely perished in the fire that ran amok down the mountainside while the boys weren't paying attention.