I may have misunderstood what you are trying to do, and if so, please excuse me.
If you want dialogue when the player faces the computer, and have different dialogue with the NPC while standing on the same tile but facing up, right or left, then,
Take the computer dialogue into its own event. Set it one tile down, i.e. on the table. Set it to 'Same as Characters' with the priority 'Player Touch'
That way, when the player presses the 'down' arrow the computer dialogue will run, and if the player presses the 'up', 'right' or 'left' arrow, the NPC dialogue will run, if that's where the NPC is.
EDIT
If the player is using a mouse, then on the first line of the computer dialogue event, if you are using Shaz's mouse script, put a comment:
<mouse arrow 0, -1>
then the player will move to the correct tile before the dialogue appears.
If you're using a different mouse script, then I don't know what you would use.