Cynthia is making an extended metaphor based on Nicolas' comment. Nicolas says that he's dotted all his i's and crossed all his t's, meaning that he's taken proper care of everything, that he has carefully done everything necessary.
Cynthia is then playing on the "writing" metaphor to point out that Jimmy uses a different set of rules: it won't matter if Nicolas has dotted all the i's and crossed all the t's if Jimmy spells things differently, which is extending the metaphor to mean "It won't matter if you've prepared everything carefully if Jimmy doesn't obey the same rules that you do."