A simple rule of thumb.............If you can see the eyes of the player putting or playing a shot, he can see you.

A solution to wazmankg's problem is to tell your line reading friend to stay behind you (out of eyesight) until you've struck the putt. Once you've struck the putt he can step quickly along the line to get the read. By doing this, you're not distracted by his position, because can't see him, and he can get the read he's looking for without bothering you. I find this works just as well on the tee box.