How should I handle movement when one character isn't engaged in combat yet?
Our house rule is that if you're not in combat or if the DM isn't having you move at your character's speed (like in a dungeon, on a trapped floor), then you can move freely and speed doesn't come into account.
I feel like many if not all people do this. Its easier to say, "I'm going to head to the blacksmith," and assume you've walked there than it is to say, "I'm going to the market," and have to calculate how many turns it will take to get there with a 30-foot move speed.
I've had a few instances where one or more players are in the middle of combat but one character will not have joined, opting to move around the battle (like 3-6 squares (15-30 feet) away and clearly in sight), but there are no enemies free to attack them yet. This character has no hiding skills or abilities that would keep them from being seen. One time, I had the enemy spot them outright, but since the enemy couldn't break away to pursue them, the character didn't see why they needed to enter the fight.
Now while everyone else must stick to their move speeds or abilities that move them around and only on their turns, this player says she isn't part of combat so she isn't restricted to those rules.
I like letting some changes to core rules happen when everyone agrees on them because different interpretations of the rules can happen, but this one always stops the game and causes a disagreement.
How should I handle this?