It really depends a lot on her interests. London has something for everyone from historical sites, to royal sites, to all sorts of museums, to world class shopping and dining. However, it is a huge city, so she needs to narrow it down to what she would specifically like to do and then Google how to accomplish it.
Personally, I would do some shopping in Mayfair, a quick walk by the palace, spend a few hours at the Tower of London, and take a ride on the London Eye, grabbing food at one of the street markets. I have spent two weeks in London on vacation and still barely scratched the surface of what is available.
I hope she has a good time!