Your friend has to go to the Immigration office and get a card (I think it is called an I-94)that will allow her to leave and re-enter the country. If she does not have that card, she will not be allowed to enter the U.S. and will have to be parolled back in to the country. (I would not wish this process on my worst enemy, It can be very time consuming and expensive, and there is no guarantee they will allow you to come back.)
She will need to go to the INS very early in the morning and get in line with all of her documentation. I think there is a charge for this card (can't remember how much) it will take the better part of the day (at least it does in Chicago, and they only serve a set number of people every day and if you get there too late, you're basically out of luck and have to come back another day). I hope that helps and that she is able to go on the cruise.