I let my neighbour use my Wifi once upon a time. At the time we had our dog who was very protective over the house (Alsatian) and used to bark at the builders who were working on block pavign the road otuside and building the others opposite.
The neighbour was a bit of a waster, lived of "Daddys" money and did nothing during the day. So our dog used to bark at the builders and annoy her - She complained to the council who in turn told us that a neighbour complained - we only had one neighbour so the process of elimination was short.
I changed the password, she then had the cheek to text me - "Is your wireless working, I can't get it". To put it blunty I told her to go **** herself.
Don't do it, tell her to get her own Wifi - Make up an excuse or let her know that you aren't going to be responsible for her actions on your own Wifi.