I think Aviatrix means "make sure your friend has your mobile number (not UKBA).

All sensible advice. I'd add that a friend of mine who was staying with me once got in a pickle at UK passport control because she didn't have my address in her cabin luggage. (She had rather foolishly put it in her main suitcase.) She was therefore unable to fill in the landing card with the UK address that she was staying at. She was allowed in, after (she told me) some discussion; it obviously caused delay and stress, so it's worth emphasising to your friend that she needs to have to hand the address where she'll be staying in the UK.