There's low probability that Marić made a significant contribution to Einstein's Theory of Relativity, let alone that she co-authored it with him.
From Einstein from 'B' to 'Z' by John Stachel:
In summary, the letters to Marić show Einstein referring to his studies, his
ideas, his work on the electrodynamics of moving bodies over a dozen times (and
we may add a couple more if we include his letter to Grossmann), as compared to
one reference to our work on the problem of relative motion. In the one case where
we have a letter of Marić in direct response to one of Einstein's, where it would
have been most natural for her to respond to his ideas on the electrodynamics
of moving bodies, we find the same response to ideas in physics that we find
in all her letters: silence. This proves nothing, as I emphasized in my paper,
but it certainly must influence our estimate of the probability that Marić made a
Pg. 36 - Google Books
(the "as compared to one reference to our work on the problem of relative motion" refers to what The Forgotten Life of Einstein's First Wife article you linked to put a lot of emphasis on)
Further down the page, Stachel also notes:
In her case, we have no published papers; no letters with a serious scientific content, either to Einstein nor to anyone else; nor any other objective evidence of her supposed creative
talents. We do not even have hearsay accounts of conversations she had with
anyone else that have a specific, scientific content, let alone a content claiming to
report her ideas.
Of course, as Stachel himself noted, this does not prove anything and indeed absence of evidence is not evidence of absence, making this very difficult or even impossible to conclusively prove.
Edit: I'll also try to address the Nobel Prize money argument. According to one source, Einstein's original intent was clearly to support their children rather than to provide some sort of compensation to Mileva herself:
The draft of the divorce agreement stated: “Disposal of the interest would be left entirely to your discretion. The capital would be deposited in Switzerland and placed in safe-keeping for the children.”
As reworded by Mileva’s lawyer, in the divorce decree the phrase “placed in safe-keeping for the children” became “In the case of the remarriage or death of Mrs. Einstein [the capital] shall go to the children.” Even if the practical consequences hardly changed – due to the fact that Mileva “shall have no authority over the capital without the consent of Prof. Einstein” - Albert’s clear statement of intent regarding the children’s heritage was swept under the rug. Yet happy to escape prolonged negotiations, in order to end an unfortunate marriage, Albert may not even have realized the difference.
Also, as noted in the comments, Einstein never won a Nobel Prize for the theory of relativity, but for his discovery of the law of the photoelectric effect.