firstly, welcome to the bf board
now to answer your ? :
it all depends on the child.
some babies have moms who drink caffeine all day and it doens't bother them. some babies have moms who have a cup or 2 of coffee in the am, and it definitely "wires" the babies during the day. some babies' moms need to toally avoid coffee, tea, coke etc, as well as chocolate (chocolate contains a sunstance called theobroimine which seems to be a stimulant as well, though it does have positive side effects too). or they are fussy and irritable all day after mom has ingested caffeine.
the only way to know is maybe have ONE cup tomorrow and see how she does...
and fwiw, use a different book for bf advice. maybe "so that's what they're for" by janet tamaro or "the baby book" br dr sears or "the breastfeeding book" by sears and sears, or "the womanly art of breastfeeding" from lll...