Hmm... well, I'm a caterer but I don't know of any places to recommend in Fiji. First of all, have her talk to a reputable travel agent. They will probably have some good ideas. If not, she will probably need to do a lot of this research online and maybe plan on getting a cheap international calling card (and remember which time zone Fiji is in) and make a bunch of calls.
Presumably, unless everyone can afford a vacation to Fiji, the guest count is going to be pretty low which should make things a lot easier and cheaper. Many hotels will probably offer "wedding packages", so that's a good place to start. If that's going to be too much money, she could ask a hotel concierge about an officiate and good, cheap wedding sites.
I have no idea what to tell you about food. Has she been there before? Did she go to a restaurant that she just fell in love with? If so, she could contact that restaurant to see if they do catering or maybe at least party platters.
Hope this helps!!!