If she's hitherto unpublished, she's unlikely to be taken on by an agent or publisher on the strength of an idea, no matter how good that idea or how good a writer she may be.
I would suggest she gets on and writes it; gets it documented before it's too late. Any writing/editing difficulties can be addressed later.
Then she could go one of two ways. She could get a copy of the Writers' and Artists' Yearbook and begin to submit the manuscript to those publishers and agents likely to represent such material (again, submission advice is out there), or she could go down the self-publishing route. I'm a self-publishing advocate, but I would advise her to consider and research very carefully before heading down that route - it has many pitfalls for the 'beginner'.
A good book to get hold of is Bob Trubshaw's "How to Write and Publish Local and Family History". It's mine of really useful information on all aspects of publishing in this genre, whether you do it yourself or otherwise.
And wish your wife good luck. It sounds fascinating.