Answer: I think it's terrific that you'll help your mother continue her gardening by making a raised bed for her. There are a number of studies showing that people can recover more quickly from various physical problems if they have contact with plants, soil, nature, etc. and that it can be an enormous boost emotionally.
I recently toured a home for physically challenged youth that incorporated gardens as part of their therapy. The raised beds were made of concrete blocks or brick with mortar, and were very basic--just four square walls at wheelchair height--but they were functional and didn't require as much measuring, tools, hardware, etc. as some types of beds constructed of wood. And you don't have to worry about using chemically treated wood if you opt to use another material.
In addition to the height of the beds, consider how wide your mom can comfortably reach across to ensure access to all the plants.
The American Horticultural Therapy Association has a website, complete with resources and information for folks in your position:
They should be able to help you out. Best wishes!
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