Hi Jamiet
Great question. These things can always be a little blurry, but we try to keep a community who feels that there is enough of a difference between acceptable and unacceptable behaviour. And like much of human behaviour, we don't have prescriptive precise language at all times. So what I'm going to do is give a continuum of actions that are OK or not OK, to help act as a reference.
Very OK
"This player always opens this way as France, I'll have to assume that's happening unless he or someone else gives me reason to think otherwise"
"I know this person often lies, I"m going to take that into account"
"This player is good, I'm going to be wary of him"
"This player is my friend, but I'm going to ally/attack with them because press/board-state"
OK
"This player is winning the tournament, I'm going to kill him regardless of press/board-state"
Grey
"This player is good, I'm going to kill him regardless of press/board-state"
"This player is my friend so I can trust him more but let's see what everyone says"
Not OK
"This player is my friend so I'm going to ally with them regardless"
"He let me solo last game, so I'm going to ally/help him solo this game"
"He killed me last game, so I'm killing him this game"
"He's attacking me in some other game, so I'm going to attack him here"
"He's dutch so I'm going to attack him"
Very not OK
You'll notice most of them come down to "Did you make this decision based on what's going on in the game, or before it even started?". Now, there's some exceptions to that: player skill, player tendencies, etc, but they all have tolerances. Intent matters a lot.
Many people have problems with the idea of rules based on intent, but at the end of the day it's a large part of what matters, and even if not tangible real, and we do our best to sort out any problems that may occur but largely rely on people being self-regulating to select out most problems.
This is enshrined in our rule:
This includes playing public games with family, friends, relatives, coworkers, or even joining a game with any player of a previous game with a predetermined intent to ally with or attack certain players. Additionally, you cannot make deals based on any factors outside of the game. (http://webdiplomacy.net/rules.php).
Players are not required to get permission or disclose that their account has a sitter.
Sitters are only bound by the normal rules of the site; there are no impositions of playstyle, effort, etc. It's up to you to find someone you trust to do so.
Sitters cannot share any games with the person who's account they're sitting.