Should I tutor a student who I know has cheated on their homework?
Whether their cheating should impact your role as a tutor is not the primary issue here, so I'll set it aside.

You should not be tutoring a student for additional money when you are also grading that student as a TA, even if the grading and tutoring are for separate courses. That sets up a financial relationship between a grader and a student. That's a big problem.

I certainly don't believe you've done anything unethical here, but you should most likely report the inadvertent conflict and stop tutoring this student immediately (I'd also offer to your superiors that you return any payment for the one session, or just do it yourself anyways). It's not a big deal that you did this accidentally not realizing the connection. It could be a problem if you continued with it now that you know.