The disconnect here is the CDC says you need a second booster to be up-to-date.
Carnival uses the same verbiage as the CDC (they say up-to-date on their COVID policies page) but they don't use the same definition of up-to-date. You only need one booster to be up-to-date in Carnival's eyes.
"Up to date means a person has received all recommended COVID-19 vaccinations, including any booster dose, when eligible."
(notice the singular plural)
However - this could change tomorrow (and we have seen their rules change overnight) so it's up to you what you recommend they do. I would tell them to either get a second booster or plan on testing two days out.
https://www.carnival.com/Legal/covid-19-legal-notices/covid-19-guest-protocols?icid=advisory_cruisehealth_040122