How should I deal with my groupmate's unprofessional behavior as a TA?
Being nice is nice, but you also need to set your own priorities. You are not the TA for that course, so you are not obliged to answer any questions. If you happen to have time and feel like answering a question (notice the singular), then fine, answer it. When not, just politely say you don't have time and that they should go see the TA. Also, you are not responsible for (bad) advice given by that TA. Instead I would focus on getting your own research done, or do the TAing of some other course.