L.J Smith is a great author, Dark visions is a good book about psycics and people with other powers. She also writes The Night World series which includes vampires, witches, warewolves and shapeshifters. You can buy individual books of there are books with three stories in them which are all linked through different characters e.g. book 1 Ash (vampire) is partially in it and he is also in book 2. Two witches are in book 3 who are in book 1 for a few pages. She also writes the vampire diaries but in my opinion they aren't as good.
Mary Janice Davidson's Undead and Unwed series is hilarious! It's a vampire book but could also be classed as Chick-lit.
Meg Cabot's Mediator Series is top! (the last one is best)
P.C and Kristen Cast write the marked series which is your average vampire book with fights and friends and love.
Has she read 'The Host' by Stephanie Meyer? Personally I think it is even better than the Twilight sega!