Yes, a prone immobilized creature can stand up.
The definition for Immobilized could seem a little fuzzy upon first reading:
When a creature is immobilized, it can’t move, unless it teleports or is pulled, pushed, or slid.
Until you note that Move is explicitly defined:
Any instance of movement, whether it is done willingly or unwillingly. Whenever a creature, an object, or an effect leaves a square to enter another, it is moving. Shifting, teleporting, and being pushed are all examples of moves.
An easy way to sum it up would be to say that the creature cannot leave it's square by it's own actions while immobilized.
The Rules compendium states this more clearly than previous rules sources:
An immobilized creature can’t reposition itself on the battle grid, but it’s not paralyzed. It can still stand up when it’s prone... (RC 232)
If you decided to rule that being immobilized prevented creatures from moving at all then you should probably also rule that an immobilized creature can't:
- attack
- defend itself (ie, helpless)
- talk
I should note that there are no conditions that explicitly prevent a creature from standing up. Neither Dazed or Restrained will prevent it. Conditions like Stunned, or Helpless would do the trick, but only because they prevent the creature from taking any action at all.
Yes you can
PHB Chapter 9: Combat, page 193, emphasis mine:
Your character can do things not covered by the actions in
this chapter, such as breaking down doors, intimidating
enemies, sensing weaknesses in magical defenses, or calling
for a parley with a foe. The only limits to the actions you can
attempt are your imagination and your character's ability
scores. See the descriptions of the ability scores in chapter 7
for inspiration as you improvise.
When you describe an action not detailed elsewhere in
the rules, the DM tells you whether that action is possible
and what kind of roll you need to make, if any, to determine
success or failure.
D&D is not like a computer game. Your player can announce various things, including "I'm trying to help him stand up". It is your job as a DM to make a ruling instead of just saying "no you can't do this because, you know, this action isn't in the list".
RAW, the PC spends their Action trying to do this. You might ask the player to make an ability check, if there is a chance of failure:
Actions in Combat
When you take your action on your turn, you can take one of the actions presented here, an action you gained from your class or a special feature, or an action that you improvise.
When you describe an action not detailed elsewhere in the roles, the DM tells you whether that action is possible and what kind of roll you need to make, if any, to determine success or failure.
I'd say it should be an automatic success in this case — the action itself is trivial, and spending an Action in combat is already a decent price. Just keep your players informed: "Yes you can help him, but you won't have time to do anything else in this turn. Will you do this?"
It should not go for the price of just the helper's movement thought — firstly, because it contradicts the rules (you must spend an Action and you can't really "transfer" your movement feet to another character), and secondly, it creates a precedent of doing something useful in combat almost "for free" (you have plenty unused movement in combat usually).
Best Answer
They Stay Prone
Stand is a simple action: it simply states:
Disrupting actions states:
And the Move Actions that Trigger Reactions rules don't specify anything that would go against the standard disrupting rules. The timing clause has implications for whether the target counts as prone when standing, which you noted in your comment. But the clear and plain reading seems to be that: