A lot of enchantment spells say that the target gets advantage on their saving throw if you or your friends are fighting it. For example, in the text of Dominate Person:

"You attempt to beguile a humanoid that you can see within range. It must succeed on a Wisdom saving throw or be charmed by you for the duration. If you or creatures that are friendly to you are fighting it, it has advantage on the saving throw. "

Several questions:

What qualifies as a friend? I assume this rule generally refers to the other members of your party, but what if you have a PC ally who only begrudgingly works with you? Or you hired some mercenaries to fight with you? Or some other third party shows up and tries to kill your enemy, but is generally ambivalent to you and your party? Is there some official Dnd definition of a friend?

Second of all, what counts as you "fighting" them? If you engage in a few rounds of combat with a creature, and you and your party decide as a group that it's not worth it to fight, and decide to run away, does that mean you can charm them as you run away? What if you've taken a prisoner who is definitely hostile to you, but you're not really fighting them since they're totally restrained? What if you later meet them in a totally different setting? What if you run away, but come back to pursue them after an hour? Is there some "time duration" after the last combat action that ends a fight?