LetLet's first state for the record that I am well aware of Rule 0 and am in no way asking a practical question. The DM's job is to make the game fun for everyone and in this regard his power is truly absolute and, when the situation calls for it, he can and should break any rule.
That being said, another DM and I have been having a bit of a discussion concerning where the DM's power "officially" ends and the players power begins. We both DM D&D 5e games with moderate levels of success but handle this specific issue vastly differently. My opinion has long been that players are completely in charge of their characters; their actions as well as their emotions. In other-words the DM only has control over factors EXTERNAL to the characters. Consequently, I would (ideally) never presume to describe the emotion a character was feeling, for example "you feel a sense of dread come over you." I would describe the situation and leave it up to the players to decide how their characters would feel about it.
My friend, on the other hand, will often describe the emotions PCs are feeling. If its a situation he feels like the PCs would feel sad he'she describes them as such, even going so far as to say something along the lines of "you feel tears begin to well up in your eyes." To me this seems like he's overreaching his power, if a PC is an apathetic robot that should be up to the player.
Is there a RAW limitation to the DM's power over Player Characters, especially regarding their emotions, and if so what is it?
Edit: To clarify spells/potions/conditions/etc that explicitly and/or directly affect the PCs feelings/emotions would fall into the External category.