Go over to the Effects forum and search on "Unibass" and "octaver" (or just "octave"). The Akai Unibass was designed for exactly that purpose, and is said to do it pretty well; and there are a number of octavers on the market which can get you in that territory as well.
Edit: I just realized you said the other
guitarist wanted to sound like another guitarist.

The fx I mentioned are mainly for making a
bassist sound like there is a guitarist doubling them or adding simple chords.
For the guitarist, I would just use a slapback delay (a delay with a very short time) set to only one repeat. Feed the clean guitar signal into that, then run the "real" guitar signal through his usual pedals and amp, and run the slightly delayed signal into a differently-voiced distortion and amp.
Or heck, the Unibass might work for him too.