Always being willing to help implies that other people help themselves when they can. Otherwise, what is he going to do when a co-worker with a real unforeseen transportation crisis needs him?
He needs to ask his co-worker when she is going to find a regular ride to work, instead of calling him every day as if she had an emergency. OP, tell him to make you the bad guy, if he must, but it is time to tell her that he is not her personal transit provider. He is married, his wife is not comfortable with the situation, and it is time his co-worker takes care of the transportation problem she ought to taken care of sooner rather than later. If her first line of transport is unreliable, she needs to find a different fall back, because she has overused this one.
Please point out that he is providing his co-worker with a convenient way to treat others in an inappropriate manner. He should not teach her that it is OK to treat people in this way. If he lets her keep this up, she is going to do this to other people, too, not just him, and it will be partly his doing. Don’t let her think that “nice people” are there for her to walk on. That itself isn’t a “nice” thing to do to others.