Perhaps not the one I mentioned, since I just spoiled it for you. :) I have found that adults don't tend to like Piers Anthony's writing (it's filled with puns and adolescent soft-porn humor), but if you want to better know where your friends - who might have read Xanth books at ages at which they could better appreciate the writing style - are coming from, I would pick the first three of the series:

  * A Spell for Chameleon
  * The Source of Magic
  * Castle Roogna

Apprentice Adept is kind of a cool dual-world series, with sci-fi in one world and fantasy in the other, and the ability for some to move between them. You should start with the first in the series, of course: Split Infinity.

Incarnations of Immortality also has the juxtaposition of sci-fi and fantasy, but in one world. The "hook" for this series is that the major "forces" in the world (time/death/fate/war/etc...) are actually offices which people fill (think The Santa Clause, but without Tim Allen or christmas). The first book of the series is On A Pale Horse.

A final word of warning: I read both of these series a long time ago, so it could be that they're actually not as mature as I remember =)

And ditch your so-called friend.