For whatever reason, at this moment Lucas realizes the catastrophic nature of the loss he has suffered. Bridge between worlds, non functional cell phone, a foreign land and foreign tongue. Although he wasn't particularly close to his parents or his sister, it does occur to him that his absence (unexplained) will pain them, and their absence from his life has begun to pain him. Friends and acquaintances come and go, but family remains constant.
Worse, from the way that this Aes Sedai speaks (a brown, maybe), no one has ever gone back. How he'd relish this adventure if it had been offered instead of thrust upon him. Perhaps the blue Aes Sedai of whom she speaks would know something more.
And so, oddly paralyzed by the wave of emotion given by the finality of the answers given by this Aes Sedai, Ferrika, and unable to express his thoughts in the tongue which might be understood, Lucas just does his best to smooth his face and his emotions, and inclines his head in what he hopes is understandable as a gesture of thanks.