Hi Laura! A trip to Walt Disney World with friends? How fun. If you would like to dine with Ariel, your best bet would be to make an Advance Dining Reservation up to 180 days in advance of your trip by calling (407) WDW-DINE.
Akershus Royal Banquet Hall is located in Epcot theme park in the Norway Pavilion. Ariel frequently makes appearances there for lunch and dinner. It's important to note that all the specific Princesses appearances are "possible", not guaranteed. Unfortunately, there is not a character meal in Disney World in which Ariel's appearance is guaranteed. You will need to have park admission for Epcot in order to dine at Akershus as well.
If your friend simply wants to meet Ariel, and dining with her isn't a "must," then you should head on over to Magic Kingdom Park in the New Fantasyland. Once you enter the Park for the day, check the Times Guide for her appearance schedule. She can be found in Ariel's Grotto near the new Under the Sea-Journey of the Little Mermaid!
Until then, I wish you a MAGICAL vacation!
-Jacqui