Regulus could feel his heart sink a little as he watched Lucius' reaction to the question. He had expected a customary answer, that of course Narcissa was fine. She had survived the war, was pardoned, but as he watched Lucius that all seemed to be a silly notion. She had, after all, lost her husband. And, as soon as Lucius pointed it out, it was obvious that it had to be difficult to see someone that was assumed to be dead, returning. More so, if the majority of your life had been spent with that person, and they abruptly couldn't remember most of what had happened. Draco's reaction, now that he reflected, had probably been remarkably calm. While Regulus was the one in disbelief about what was happening, if anyone else had found him, they probably would have called him the liar, the one that was playing the trick, setting the trap.
He felt a distinct amount of gratefulness for Draco at that moment, but was also saddened at the prospect that he would probably not be able to reveal himself to Narcissa, one of his beautiful older cousins, for quite some time.
The emotion that he felt seemed to weigh double on Lucius. His usual composure was slipping a little, so that even Regulus could see the pain that Narcissa's less than happy welcome and retreat from society had brought him. He was uncertain what to say in the face of this revelation. He wanted to ask about the rest of his family, but was scared that the answers would be the same, or even worse.