The figure standing outside was a close friend of Nathan's, and he was there when Grey embarrassed him.
If Nathan had any ill will towards Grey, then this young man also did the same. And unlike Nathan, had he been the one who found Grey, he would've killed him without a second thought.
Alec was not only protecting Grey, but he was also putting himself in harm's way. He was still in the Third stage of the Sage Plane, and these people were all in the Mid or Late stages of the Sage Plane. If Nathan really wanted to, he could force him to leave Grey alone and kill him, but knowing that he couldn't kill Alec, he didn't take the risk.
This guy, on the other hand, had a very powerful figure amongst the top Elders, and there was a chance that he might be able to get them to help him out if he caused any trouble. Besides, it is common knowledge that a dead genius was useless.