how do i stop my friend from commenting on another wall post on facebook?

[abraXus]

there is no way to do this

if you set up the post ahead of time so only certain people can see it, that would have prevented it, but now that both people have left comments on it, they will both continue to receive notifications every time a new comment is left on it

your best bet is to just delete the post entirely, and communicate with the person privately using messages instead