I haven't ever asked him why he thinks that. I would say that if you are traditionalist and you know everything the Novus Ordo stands for (bugnini, pariti, ottaviani, etc), then you wouldn't go because it could possibly be a sin of commission with regards to the complete rupture with tradition and all that the NO stands for.
Also there are virtually ZERO novus ordo masses that are celebrated according to the rubrics, since there are so many options. Not to mention all the abuses that take place, by attending one is basically saying that they're ok. I think when he says it's a sin to go, that's what he means. That basically there is no such thing as a well done Novus Ordo.
I don't think he would say in and of itself, the rite itself, is evil, although I don't know.
I have heard both sides of the argument, whether it in and of itself is evil, that is promulgated by the church; therefore it cannot be evil.
I honestly do not know, I think there is a possibility that is was not promulgated correctly, and the only thing making it valid is that they use the correct formula for sacramental validity, besides that, I'm honestly just not sure. I suspend judgment with regards to that particular aspect, I just avoid it.
I haven't had to attend the Novus Ordo in a loooong time, and I'd like to keep it that way.