According to Carnival's site, your friend won't be able to use his/her own laptop to directly connect . Here is the information from their site re: cost:
"Internet service may be purchased on a "Per Minute" basis, or there are two "Block Packages" to choose from. The "per minute" charge is $.75. However, you may choose Block Plan 1 at a total cost of $55 for 100 minutes ($.55 per minute). Block Plan 2 is $100 for 250 minutes ($.40 per minute).
Charges for this service will be billed to your Sail and Sign Card. "