bkc
I have not completed a "carved" pen yet, but have a few drawings, and experience with pen turning, and carving.
Here is what I "suggest" and you can maybe get some ideas from these:
1. In the Sylus pen magazine there are pictures of pens that have been turned to the dimensions of the bushings RIGHT AT THE POINT WHERE WOOD MEETS THE PEN KIT, and then slope out to a design that is fairly substantial, one of the barley or rope twist pens with deep flutes comes to mind. The flutes taper out at the ends to keep a nice "flow" into the bushing dimensions.
2. Also there are some that are turned like above, but about 1/2 inch from the point where wood meets kit, a very large "step up" in dimension occurs to allow more "meat" to be on the brass tube in the middle of the body or top. I consider this a carving field in order to do the work you need to, and the thinner stuff is just there to make the kit work and act as a frame for the carving. How you handle the "step up" and transition is a matter of taste and style, but there are some very good example in STYLUS of these.
3. Several of the jade carved pens use some or both of the techniques described above.
Hope that helped and didnt create even more confusion. If your buddy is an experienced carver, you may want to remind him how "challenging" carving Osage Orange will be. Granted some Osage is not bad to deal with, but some of it is very difficult to carve, and I would recommend dental bits and a high speed power head. I would love to see what you come up with, and see your friends handiwork when he is done!
Robert