I would say no, they won't fit 'properly'.
If your mate has an R33 GTR and the wheels sit 'nice' on it, I'm assuming they're at least 9.5" +12 offset. From what I've seen, R34 GTt's running standard R34 GTR wheels, they were right on the guards if not a little out. So if your mate's wheels are 9.5" +12 (assuming somewhere around there because you said they fit nicely) no, they won't fit.
Why not trial fit them to see for sure?