Pardon the longwinded response.
Hypersmurf said:
She's already initiated the Cast a Spell action, though - it's what triggered your readied action. She can't take back that standard action and make a full-round Withdraw action instead; if she weren't casting a spell, your readied action would never have come up.
She could perhaps abort her casting (losing the slot) and take a move action, if she has a move action left over.
And that makes sense? You can't "take back" the casting and do something else, but you CAN just not cast at all... which would by rights not trigger the readied action just the same?
Readying actions is freaky because 3E+ has done away with declaring actions and then being FORCED to follow through. You take whatever actions you decide when your turn is up and whatever happened prior to your turn NOW is largely irrelevant. But you still need some kind of mechanic for being able to shoot at the first thing that comes through the door without having to wait for your initiative - to interrupt what others are doing on their turn. It does lead to some "time warp" moments but you can't avoid that without breaking down the 3E system and starting over from scratch, building in lots of conditional-completion mechanics for various actions. You START to move, but it was a trigger for a readied action so how much do you actually move? You START to cast, but it was a trigger for a readied action so DO you actually cast?
Example: I'm in a 5' wide corridor and see an enemy some distance away. I ready a move action if he takes a 5' step towards me. He starts his turn with a 5' step towards me. My readied move goes first and my initiative changes. I move up to the square right in front of him - clearly preventing him from moving that 5' step towards me. But does that mean that when it then becomes his turn again he MUST move 5' forward even though he can't? Does it mean he can't move at all? Does it mean that he can't take a 5' step backward and attack with his reach weapon since he had intended to move FORWARD and attack me with it before I ran up to him? That by closing the distance I render him unable to do anything since he had INTENDED to move forward? Does it then matter if he INTENDED to cast rather than move?
It doesn't matter that he actually complete the action he started that triggers my readied action. Unless something I do has an overt effect upon his ability to act when it is his turn he acts as he WANTS to act.
Another example: I ready a move action to run into the midst of a crowd of orcs when my friend the wizard starts casting. He starts casting. I run into the crowd of orcs. Now he had INTENDED to cast Fireball on the orcs. Does he still have to do so? Can he not cast Haste on me instead? Of course he can because he gets to decide what to do on his turn based on everything that's happened up to that point - including my interrupting his originally intended actions with my own actions.
There are specific provisions described in the Ready action for INTERRUPTING a spellcaster to disrupt what he's casting - but then you Ready the action that WILL do so. If I start casting, you take a readied move triggered by my having started casting there's nothing in the description of Ready that says I MUST finish my casting, that if I do cast it MUST be the spell I had intended to cast when I started, etc. Regardless of what triggered your action, when it's my turn I get to act however I want based on all events up to that point.
One more example: You have an action readied for when I start casting. I start casting and your readied action is to say "Boo!" I now have to FINISH casting because you said "Boo!"? Ready exists to interrupt and prevent others from taking actions, not to COMPEL them to take actions.