I don't know of any text that mentions Jill's help for the TeX
language, the implementation, etc. But if the question is extended to
the complete project TeX, METAFONT, and Computer Modern her help is
documented.
Besides all the support that she gave Donald Knuth in their
relationship---the NYT article mentions that he starts to be active at
night, sleeping during the day; what does that mean for the
partner?---he mentions a couple of times in his papers her
contributions.
First in the article based on his his Gibbs lecture of 1978, in which
he talked about his research in Mathematical Typography. The section
Acknowledgments starts with the words
I would like to thank my wife Jill for the many important suggestions
she made to me during critical stages of this research
(page 59 in Digital Typography (DT)). Earlier in this article he wrote
With my wife's assistance, I finally came up with a satisfactory
solution
In this part he discusses the problem to draw the letter `S'; page 48 in DT.
On page 64 of the book Companion to the Papers of Donald Knuth he
says
Jill helped me at this point: She took 35mm slides of the master pages
that I'd gotten from Addison-Wesley
That was an early stage, in which he tries to get the letter forms used in his books The Art of Computer Programming.
plain
) is something for recipes, designed with her input and for her to use (see "Recipes and Fractions" and "Macros for Jill" by DEK in TUGboat) -- some more of TeX's features came about from such applications (contd)