There are no hard and fast rules as far as visitors and money. If she doesn't have a hotel, £50 a day would be more then enough. You can get a skewed sense of reality on this board and we scare people a lot...
Having a sponsorship letter is a good backup, saying that you will accomodate and feed her for the two weeks. Having some cash in hand is a good backup. She should also have a way to contact you, mobile number. She should know where you live specifically, including the address. The most important thing is being clear and concise. Always tell the trust, but only provide the information being asked for.
What is likely to happen:
IO: What is the purpose of your visit?
GF: To visit my boyfriend?
IO: How long will you be staying?
GF: Two weeks?
IO: Do you have a return ticket?
GF: Yes, here (hands over itenerary)
IO: Have you met your boyfriend before?
GF: Yes.
IO: When was the last time you saw him?
GF: [Whatever is the truth]
IO: Thank you, enjoy your stay, *stamp* *stamp*
If she is unclear, hesitates, says "just visiting a friend", appears to be evasive they are going to grill her more, maybe go through her stuff, potentially attempt to trip her up, maybe call you to verify her story, etc.