Some cruielines are now requiring passports even though the ports visited do not require them. I heard that this is because the cruiselines believe that there is a strong possibility that the countries involved may change their passport requirements in the near future with very little notice. Remember, the country involved can set any requirements it wants reguarding identification requirements.
Other cruiselines are strongly recommending passports, but have not yet wanted to make it required.
I believe that Cozumel requires a passport for visitors staying on the island but does not yet requires it for people in transit (staying less than a day), but some believe that this could change at anytime.
Some believe that in the near future, passports will be required in most countries, it is only a matter of time.
Her cruiseline is the final authority on what she will need to board. So if her cruiseline is not requiring a passport, she should be OK, but there is a reason that they are recommending it.
Times are changing, I'll second the cruiselines recommendation.