It has been argued by many that a 6speed is not worth the trouble on a Z32. The engine and diff are designed to produce a power band that matches the 5 speed transmission.

I've never seen anybody attempt to mate one from a RB26 to a Z. I have seen ideas about swapping in a T56 transmission from a Dodge Viper or Camaro. The 350Z 6 speed has also been mentioned, but it apparently will not fit.

Per Tom Bell regarding swapping in a 6 speed transmission:
"The tranny won't bolt up directly. The mounting plate isn't the same pattern, and the input shaft is a different diameter with a different spline count. The input shaft length to the tranny front cover is different as well, and the tranny itself is longer, with a different mounting point. As Mike Smith points out, the starter's also on the other side, which may interfere with the steering box."

In addition to that, I am pretty sure that some custom parts will need to be fabricated to make it work including (but not limited to): driveshaft, bell housing, clutch, rear tranny mount.

If your buddy can afford the 6 speed transmission and has the money and resource to produce the customization that would be required, he would just be better off getting the Holinger H6S (Holinger makes a 6 speed sequential gearbox for Z32's and Skylines). I believe the transmission runs upward of $10K and Tomei makes a bell-housing adaptor for it which runs @ $1100.

Good luck...seriously.