I think the most significant quote Darry makes about Ponyboy and the deaths of Dally and Johnny is found in Chapter 12. In this chapter, Ponyboy is refusing to admit the truth about what happened, and is living in a haze, not really caring about anything. Darry is getting after him to attend to his schoolwork, at which he always excelled, and Ponyboy rebels, telling Darry that school is not a big deal; he'll just have to get a job when he graduates anyway, Darry replies,
"You're not going to drop out...but schoolwork's not the point. You're living in a vacuum, Pony, and you're going to have to cut it out. Johnny and Dally were our buddies, too, but you don't just stop living because you lose someone...You don't quit!"
Darry acknowledges the tragedy of Johnny's and Dally's deaths, but he rightly emphasizes to Ponyboy that life goes on. His eloquent plea is what Pony needs face and accept the truth of what happened.