" it would be different if she asked you to score drugs - well she is in a way, isn't she? what other purpose do they serve?"
They are legal, though, so the OP isn't compromising herself, unless she follows a religion that forbids smoking.
OP, is she on the benefits that she should be on? and does she need help that you cannot provide?
In other words, does she need referring to adult/disabled SS?
She has enough loss of control, by the sound of it, it is wrong for her choices to be cut down down further. I am sure that we could all look at each others spending and nearly halve it (i am vegetarian, for example).
Look at all of the threads on here about the posters parents having the cheek to have disposable money and go on holiday with it, when those posting don't. Travel the world and discover that compared to many, we live like kings, in the UK.
I would be honest and tell her what i think, though. You are not obliged to help her out with money, i think that, that is the problem. You are in a situation and she is taking you for granted.