Andrew Lynch, you're right about the cats, although goats are actually becoming more of a problem. When I was there in '99 they actually had specially selected people who were sent out as goat snipers to some of the islands. Machetes aren't much use in the Galapagos since you're not allowed on land without a certified guide, who take offense to the hacking of the local flora and fauna, also there isn't much other than scrub and a few small trees on many of the islands, at least in the Southern Islands. I wasn't in the Northern Islands they may have different vegetation.
Mark123 your friend should find out about Anti - Malarials, I know we had to take them, but that maybe only because we passed through Guayaquil on the way out, which is in a malarial zone. If she does have to take them I recommend the tetracycline based ones, they're cheaper and the Mefloquine types can cause hallucinogenic dreams/nightmares (They call them 'vivid dreams'. I would recommend a good canteen, and either iodine pills or a purifier. Most water provided by tour groups is probably safe, but it's cheap insurance. Others have already suggested a good camera, I would add that good binoculars and a small first aid kit might not hurt, as would any American types snacks which she might miss such as Granola bars etc...
A good headlight or minimaglite might be a good idea too. Some of the islands have awesome lava tubes that the guides will take you into.
Hope that helps a little, sorry for the long post. I'm jealous, I would go back in a heartbeat if I had the time and money.
Lagarto