Surely some undergraduates will respond to the motivation of reading famous papers, but I'd usually recommend motivating them with ideas or applications instead.
1) What they expect in the paper may not be in the paper. Your student probably wants to see $E=mc^2$ from the time Einstein understood this. They may be disappointed to learn that Einstein's closest paper from 1905 said only "Gibt ein Korper die Energie L in Form von Strahlung ab, so verkleinert such seine Masse um $L\,/\,V^2$", or
"if a body gives off the energy $L$ in the form of radiation, its mass diminishes by $L\,/\,V^2$."
2) Many famous papers are hard to read. Often the first presentation of an idea is messy. Often after an idea is recognized as noteworthy, people start talking about the idea in a different context, so that the original context seems odd and difficult to understand. What was common knowledge at the time the paper was written may not be common knowledge now, or may not be known to the student.
3) Even if they succeed in reading the paper, they may not be able to answer basic questions like: What was new in this paper? What similar things were known beforehand? Answering those questions requires studying the historical context, which is hard. Most profs would have difficulty providing accurate historical context for most famous mathematical papers.
However: if they want to read some historical papers in an area (rather than one particularly famous paper), there are lots of sourcebooks with accessible selections and helpful introductions. I recommend those sourcebooks for studying the history of math, rather than fixating on one famous paper.