My advisor asked me to co-author a paper, but I have no interest in an academic career
Congratulations! In my opinion, it is perfectly polite to write back and say that you are interested, but can't afford to make a significant time commitment to the project. Ideally, the professor will be happy to do the work himself, and your role would largely be to look over the finished product, sign off on it, and make suggestions or comments.
It's fine to invite the professor to write this paper without you as a coauthor. That said, since you did much of the work already, the professor might prefer that you be listed as an author.
If you are thinking of possibly applying to graduate school in the future, it would probably be a good thing to have a publication under your belt.