Things like paintstripper or spray paint spring to mind whenever I get ripped off by so-called mates but I've never acted upon it. I think the best thing to do is just confront her and ask her when she's going to pay you back. Don't even give her the chance to deny it, just say you know that she's talking nonsense, you had �200 then she knicked �40 and left you with �160, those are the facts, she's shouldn't have done it, you shouldn't have to subsidise her poor ability to look after her cash, she should pay you back when she can afford it. She should aggree or start crying or something. I would regard crying as a total result in this instance for you, short of getting the cash back, an apology and or admittance of guilt is about the most you can hope for.