One of my friends got an on-site interview opportunity for a junior quant position with one of top banks on the street. He actually just started his MS study in Fin Math a few months ago, expects to finish next year. He doesn’t feel that he is now prepared for any real interview as he is still getting his hands on financial mathematics through his course work. He has some good programming experience which might be the reason he is invited to this interview. I don't know if it is wise for him to go to the interview and potentially waste the opportunity. I have heard that if you didn't do well in the interview, you may end up leave a record with their HR system and would have lots of trouble to get another interview from the same bank later even for position at a different desk.Wondering

I'm skeptical of this. Banks and large corporations are extremely decentralized creatures, and groups are extremely jealous over their ability to hire people independently of what HR thinks. My experience is that any bad consequences from messing up your first interview is far outweighed by what you learn in going through the process. It's part of your education.