Assuming that changing her typing style is out of the question, I'd recommend looking at cushioning rather than the keyboards themselves. It sounds like she's having the same problems I had when I moved from a mechanical typewriter to an electronic keyboard: where once high pressure was both necessary and comfortable, it now becomes neither. The forces used to push the keys aren't absorbed by the machine, so the pressures causes stress, inflamation, and pain.
My first step would be to put cushioning under the keyboard. The easiest way to do this would be with a standard keyboard placed on the lap, where the legs act as the cushion. This allows the keyboard to sink, absorbing excess pressure. Alternatively, a pillow or two wrist-rests could be placed under the keyboard. With a laptop, this might make the screen shake too much, but this can be avoided with a cheap external keyboard.
(Windows tablets, by the way, tend to be compatible with standard USB devices like keyboards. I can't say I'm happy with Windows 8.1, though: the "apps" environment crashes with startling reliability. Desktop apps work fine, though, if slowly with my Thinkpad Tablet 2. The problem with tablets is finding a stand that works well for keyboarding; I mostly use mine for handwriting and content consumption.)
I wouldn't recommend an ergonomic keyboard to someone unless they were willing to adjust how they type. If she's willing to adjust how she types, then using less pressure is the first step, not a new keyboard.
As far as actual keyboard hardware goes, though, I couldn't be happier with my Cherry clears. They have a clean break, like a good scissor, but then they have a nice amount of resistance that makes it easy to learn how much pressure is necessary to actuate the switch without bottoming out. They don't make much noise, either.
Of course, it could be more related to the angle of her wrists when she types, but that requires her to have more awareness of her environment, which isn't available when going to a coffee shop to write. (I'm a writer myself, so I understand the coffee-shop aspect of things.) If she's willing to submit to our analysis, I'd ask you to take a picture of her forearms and hands on the keyboard while she's typing, which might help us say "the keyboard should be closer/further and higher/lower."
Finally, stretching the wrists intermittently during long bouts of typing is rather effective at improving comfort. Hold one arm out straight, then use the other hand to alternately pull back on the fingers when the hand is up and palm out, and then to pull back on the knuckles when the hand is down and palm in. Do the other hand the same way. Rotate each hand in figure eights. Push and pull the thumb, too. Repeat as feels good. It may look silly, but it can work wonders.
Cheers,
Jon
