How can I teach someone not to keyboard turn?

Practice and concentration on the new controls. It just takes a while (3 weeks?) for something to become a learned behaviour.

It might help to avoid stressful situations at least during the early part of this period. When stressed we revert to what we think we know best. This is why you wash your windscreen when you meant to flash your lights when someone cuts you up when driving as your previous car's controls were the opposite to the one you have now.

So if you are engaged in guild raids - which sound stressful - your wife is more likely to revert to the previously learned behaviour (keyboard turning). With enough non-stressed practice at looking with the mouse she should be comfortable with it when it really matters.

Just to add something I posted as a comment on another answer:

Minecraft can be played in peaceful mode with no enemies and has the "classic" keyboard and mouse controls so might well be a suitable training environment.

Change her key bindings for a month. Re-bind A and D to strafe left and strafe right. Without a key allowing her to turn, she will be forced to learn to mouse-turn.

If you'd like an example of how powerful this is, try Sindragosa hard mode. Every keyboard turner I see ends up dying every attempt because they can't get behind the ice blocks during the air phases.

The easier way would be to let her play a game that doesn't allow you to turn with a keyboard (or at least by default), something like a FPS for instance.

That way she's forced to learn the new controls, in a 'fun' way.

As soon as she get's the hang of it, it will be easier for her to use it in WoW too