Ilya is always aware of David's movements. Not in an intentional way, just in an instinctive one. He's the first person to realize that David's hands aren't as steady as they used to be, that it takes him a little longer to catch up, that he has to actively try to keep up with Yuna and Shane.
He's absolutely terrified because he's seen this before, the far-away, glazed-over eyes of a mind slipping like sand through his fingers. He has stood in front of it, trying to get any sliver of recognition, to fight the dying neurons with the best pills money could buy. His father wasn't a good man, but he didn't deserve to have his essence erased like that.
David drops a knife and Ilya is the one that reacts, cutting his hand in the haste to get it, and while David is patching him up, Ilya is watching him. Maybe if this time he does it right, catches it early, maybe it won't be able to take David away from Shane. From Yuna. From him.