Well if she is not doing an excursion, there is a Margaritaville in Montego Bay, (maybe a mile to a mile and a half from the port) But she will need to take a taxi to get there. I'm not sure I would recommend doing that alone being female. (If she tries tell her to look for a female taxi driver.)
There is not much she can do "on her own" in Monego Bay and I would suggest that she take a ship tour (despite the cost) if she wants to see some of the island. Some of the "beach break" or catamaran tours are not very pricey and that at least gets her off the boat safely. Just make sure she can answer with a firm "No Thank You" a million times without caving. I always will remember the 60 year old man with short gray hair coming back onto the ship with cornrow braids... obviously a guy who either lost a bet, or just got tired of saying "no"
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007