How bad are the tremors? Maybe something with a keyguard could help. A keyguard is a piece of metal or plastic with holes cut out above the keys - the idea is to reduce the number of keying errors if the user can't control their hands 100%.

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Her typing speed will go down, but the increased accuracy might make up for it.
If you think it might help, you'll probably find it easier to buy a keyboard and guard as a set, than to find one to fit her current keyboard.
[edit - sorry should have asked about the Parkinsons before wading in]
Also, you might want to consider text expansion software. It's like autotext in word - but works anywhere - including browsers and email programs. So if she types msvt; - that could be a trigger for "my son visited me today". For best results, she needs to create the triggers herself, and just a few of them each day.
Try to encourage her to take a break halfway through an email.
Keep in touch, I hope we can suggest something that can help her.