I don't fish lakes a lot, but when I have, it seems like the movement/retrieve that you mention is more important than the specifics of the fly itself. Keeping the nymph at or near the bottom is more critical in moving water where the fish have to hang there for the cushion it provides from current, but that dynamic doesn't exist in still water obviously. I would try a soft hackle in a gray-ish color, roughly that size, though I doubt it has to be exactly an 18 either, and then experiment with different speed and cadence retrieves. You should be able to look over at your friend and get a pretty good idea of how he's retrieving the fly - if he's catching fish, I suspect that would be more valuable to copy than the exact fly in question.