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?
Best Answer
You can't freely change time and space for only one player
I never GM'ed, but as a player I fully agree with Kyle Doyle's and Novak's answers. Your player shouldn't move freely.
If you're looking at it logically, moving freely (even by your rules) means that your player can act several turns at once (and then you calculate how many etc...). But if only one player is able to do this while others are "restricted" because of combat, that would mean that following your player's logic her/him could play several turns while others can only do one, which is illogical. If your player wants to move while others are in combat and monsters as you said can't attack her/him or anything in touch with the combat would affect her/him, I would follow the suggest of Novak, which is to say "Okay, but this will take you N turns to get here" and state her actual position each turn (maybe she can't move a bit further than someone actually fighting as it's her/him only action, it's up to you).
In RP I already played, displacements are like in yours (but we don't really care about rounds/turns when we're in town for example, without combat we're more speaking about time spent). And if the party's split and one part is engaged in combat, if the rest of the group is aware of that and wants to join them, then our GM tells us how many turns we'll have to wait before actually being in combat (sometimes with dice rolls if there are particular conditions to join them, like lots of people in a town or things like that).
Malacandrian posted a new answer while I was writing mine, but I also fully agree with this one. This answer's approach to explain how turns work could be helpful to you to argue with your player that she/he can't do it.