I actually lost 6 lbs on an 11-day cruise once. You can follow any diet plan while on vacation, as long as you know what is okay to eat.
If she wants to count calories on the trip, she's going to have to spend some time doing so. There is a diet board that has a great calorie tracking program, here: http://forum.lowcarber.org Go to the "My Plan" link. It requires some innate knowledge of portion sizes (ie...did you just eat a cup of soup or a cup and a half), but she should be able to track her calories with a reasonable margin of error. She is going to need to keep a little notebook with her to write down her food (including the types of sauces, etc that come with it).
I think calorie counting will be possible, but a giant PIA. A better approach might just be to only eat foods that shouldn't be a problem calorie-wise. Eat a lot of fish (other than fried) and other seafood. Eat chicken, lean cuts of beef (like tenderloin), and veggeis. Skip sugary drinks, desserts, breads, and "junk food" like fries, nachos and pizza. Instead of dessert--have fresh fruit at the end of the meal.
CSA - May 2012
CN - May 2013
CSS - October 2013