The title was probably kind of confusing. But you know how Pop-Up GUI bricks only work if the player touches the brick? Well, is there any way for an NPC to touch the brick and force a GUI to appear on the screen for the player?
What I would do is have the gui already set in the player's screen but disabled, and have it when the part is touched by another part, check if it is an NPC, if so, Pop-Up GUI gets enabled.