These are all solid, basic books that range from early books through what is called the Golden Age of Science Fiction, and a bit into our own age also.
In no particular order, other than how they occurred to me, here are some classics (and a couple of extras) that I love and hope will usher my friend into enjoyable reading.
- I, Robot - Isaac Asimov (short story collection)
- Journey to the Center of the Earth - Jules Verne
- The Invisible Man - H.G. Wells
- Alas Babylon - Pat Frank (credited with being the first apocalyptic novel)
- The Mote in God's Eye by Larry Niven and Jerry Pournelle (featuring my favorite idea of an alien species ... very, very alien)
- Agent to the Stars - John Scalzi (light and easy, featuring very friendly aliens and Hollywood)
- Citizen of the Galaxy - Robert Heinlein (my favorite Heinlein)
- Fahrenheit 451 - Ray Bradbury (arguably Bradbury's only science fiction)
These recommendations are for someone who has never read any science fiction before. There are tons of good books out there, but I'd argue that many of them are at least what I'd call "second tier" books ... those which might actually turn someone away from the genre if they haven't read anything else in it.
I was also trying to be fairly specific about science fiction versus fantasy, with which my friend has a good acquaintance already.
Science Fiction: fiction based on imagined future scientific or technological advances and major social or environmental changes...