Take out the money for another advert and a pint or two and throw the rest back at them if they make any funny squeaky noises. At least you found out now that she's not really reliable. Its not worth the hassle for a few quid IMO.

I don't mind having a lodger as I've got the space to, but I certainly wouldn't let anyone in the house that wasn't either someone I knew or a friend knew. Sod putting locks on doors, etc...