There is a millisecond when the glider's blades are racing towards him that Peter can save Harry.
He can kick out at his friend and send him crashing to the ground and safety.
In his dreams this is what he does. Harry falls away as the sharp blades penetrate his own chest. It hurts, a terrible ripping pain, and he can feel as his body begins to shut down and his heart pounds unsteadily against the damage. He thinks, looking at Harry - safe and alive, that this is worth it and this is right and this is what should have been.
When he wakes, he's still alive and Harry is still dead.
The pain from the dream remains with him though; a horrible burning ache that never seems to leave his chest.