Yay - good for you, and I think you handled it perfectly. You didn't go into any long-drawn out, specific reasons why you can't do this anymore, but you simply said, with a vague explanation, it won't work, and you have to find another way. If it causes her to stop working, that isn't your problem. Its HER problem to get to and from work. Not anyone elses and if she really wants to continue, she will find a way.
I posted some months back about a CW who did not have a car, lived within walking distance of work, but would always ask for a ride home. And I didn't think that was right for her to do. You take a job, knowing you can walk, you walk. While I wouldn't have any issue giving her a ride home if the weather was nasty, but if you take a job, knowing you don't have transport, its your responsibility to get there and back, and not ask everyone for rides.