The most frugal approach might be to consider a trackball, followed by a full computer drawing table (e.g., http://goo.gl/qlHnlz or equivalent). Depends on the nature of her tremor or shake. Alternatively, get her a gaming mouse and turn the dpi sensitivity all the way up such that any movement is translated into minute screen movements.
There are dedicated hardware vendors, found on rehab sites such as http://www.rehabmart.com/product/adapted-wireless-computer-m... that you might consider browsing. I had a link to a product that seems to have gone out of business - it was an adapter that autoaveraged input coordinates to tone down tremors.