It seems to be unanimous opinion with everybody here to get this lodger out ASAP ,so take smart advise and just do it. All the talking in the world wont change anything and the longer it goes on the more money/patience you lose.You know you have been taken advantage of as does everybody here and she knows she is getting a cushy deal so now is the time to stop being soft on her.So you lose a lodger...no big loss when you consider that your not getting what you wanted/needed out of this deal.Your certinatly not losing a true friend...anyone who does what she is doing and expecting this much out of someone else regardless of her suitation e.g Exams, is not a nice preson and not to be trusted never mind calling them a "friend"! Its your house/money/life at the end of the day and as hard as it might seem or as bad as you might feel now, in the end it makes sence and might give you the confidance and knowledge if you where ever faced with a similar suitation.Let us know how you get on.
P.S
Never put off what you can do today, untill tomorrow!