Joined friend, can't take quest without him?

Traders give quests to individuals regardless of where their allies are standing. If the trader wouldn't give you a quest until your ally was nearby then you are probably playing a modded game.

Shared quests will give you a warning at the quest marker that your ally will fail the quest if he is not close enough. But you can still start the quest without them and they will simply fail it and you can do it alone.

The default game does not force you to quest with any of your allies. You can do quests without them. If they fail the quest it is just a message but they don't lose reputation or suffer any penalty at all.