I will of course give her 2 or even three weeks to find another ride.
I'm not the OP, but my guess would be:
-because she wants to minimize the impact on the coworker
-because she has to continue working with this lady and wants to maintain as "nice" a relationship
-to appear to external parties (fellow coworkers) to be fair and reasonable and minimize any workplace unpleasantness which while shouldn't happen in reality might, people being as nosy and rude as they are
-because after letting it drag on for so long she feels bad cutting off cold turkey (she feels bad enough cutting off at all, it's harder if you feel you haven't even warned the other person).
Of course, none of these things are necessary and the OP is not required to give the lady any notice, but I completely understand why she feels it's something she wants to do. I don't think there's anything wrong with that if it makes her feel more comfortable with her decision.