Edmund has been Eustace's roommate for a period of time, so he knew him quite well. Even before they had their adventure in Narnia – back when he still hated the other lad – he had observed his cousin's mannerisms and actions. One particular thing he observed from his cousin was the way he stared at things.
The Just King would chuckle at the way his cousin's eyes glow whenever he sees something fascinating or something intriguing. Once, after their trip in Narnia, as they were walking in the park, the younger lad's attention was caught by a peculiar insect. Edmund did not get what the fuss was about. There was another time when Eustace would just gaze upon the clouds above. Edmund had to listen to his blabbering about "Look how beautiful the clouds are, cousin!" and "Surely the skies here are nothing compared to Narnia."
At those times when he has been around with Eustace, Edmund realized how his cousin would appreciate even the simplest and smallest of things. He also observed the younger lad's eyes – how it just pierced through everything it would look upon. It's as if Eustace could see beyond every detail. He would stare at something and would not look away – especially at beautiful things.
Edmund came back from his reverie and into the present. He was looking at his cousin's constant stares at something that once again fascinated him. Only this time, he was staring at something much more different than anything that ever intrigued him before.
The older lad was leaning against the tree, while the other one sat on the grass. From his point of view, Edmund could see that his cousin was staring at the young maiden from a distance.
"Why don't you just tell her," he said suddenly. It came out more like a suggestion rather than a question. Edmund just couldn't help it.
Eustace turned to look at his cousin. "W-what?" he asked, confused.
Edmund sighed. "I mean, why don't you just tell Jill?"
"Tell P-Pole… what?" Eustace's confusion grew even more, but there's something more behind his reaction – something Edmund saw.
Edmund's lips twitched slightly upward. He saw it. He saw how Eustace hid something behind his obliviousness. He saw how the younger lad concealed his nervousness with so much effort. And he saw, just about a minute ago, how his cousin's eyes looked at Jill Pole, as if she's the most beautiful thing he has ever seen.
"Tell her…" The older lad sighed and looked up, a smile on his face. "How beautiful the clouds are."
Note: Because it took me this long to actually finish The Chronicles of Narnia, and I fell in love with Eustace/Jill. Oh, and Edmund. I love Edmund. :)