They can certainly publish in top journals. Usually the postgraduate student is the one who did most of the work and is the first author on the manuscript. But the manuscript is expected to have a senior researcher (e.g., the supervisor) as the senior/corresponding author who is responsible to submit and communicate with the journal. Now, judging from your follow-up question, you refer to the situation where the postgraduate student acts as the corresponding author. This might be a problem for the top journals. I've known a lot of postgraduate students who had no problem doing that with very good journals in their respective fields but I've never met anyone submitting to the top journals as corresponding authors.
Yes of course, the only problem can be source of money for publishing it. He can even send his paper to journal. After review stage when paper is accepted there is again only question who will pay. :-)
They will publish a good, solid, but traditional work. But they will reject the brilliant work that opens up new approaches to science and technology and expands the horizons of our civilization. And this is due to the lack of intelligence and stupidity of experts and reviewers