Grandpa is pathetic for having lied to that kid. What's he going to say later? "Oops I felt bad, awakward?"
"Oh, your friend died bleeding while stranded on the cliff." . Then, the kid lost his will to live for leading his friend to his death.
Do you want the Grandpa to rescue the rest of children while having the extra hands to assist in his attempt?
Or do you want the kid go either berserk, getting himself killed or become depressed and kill himself?
The kid has an inkling about his friend demise, but choose to ignore for the sake of surviving for the next day.