Many good tips so far in this thread. If your friend reads a lot of papers, they might be able to "leap" into classics without being bogged down by the language and sometimes dense prose - but I'd still err on the side of accessible for getting someone into reading for pleasure.

I'd recommend for example (in no particular order - just some great books imnho):

Earthsea quartet by LeGuin

Dune by Herbert

South of the Border by Murakami

Islands in the Net by Sterling

Foundation trilogy by Asimov

Speed of Dark, by Moon

Accelerando, by Stross

Murderbot diaries, by Wells

A Fire Upon the Deep, by Vinge (and Rainbow's End by same)

2312, by Robinson

And many more, I'm sure :)