Antler is bone. Antler is not the same material as hair and finger nails. Horn, on the other hand, is the same material as hair and fingernails. What is the difference in antler and horn? Antler is shed each year and regrows and horn is permanent. Deer and elk have antler and cattle, sheep, goats, and the like have horns. Yes, we call them deer horn but they are antlers, not horns.
Different species (that have antler) have different amounts of bone marrow inside and solid bone on the outside. Elk antlers have lots of marrow and little hard bone. Whitetail and mule deer have more solid bone and less marrow in their antler. Axis deer antler is my favorite. Axis antlers are straight and solid bone with very small vein of bone marrow, often about the same diameter as #2 pencil lead.
Yes, antler is hard but does turn quite nicely. And yes, there is an odor. To some t it is an awful smell and to others not so bad.
Do a youtube search and you will find some videos on making pens with antler and how to drill antler.
Good luck and your friend will love his pen. I have made many antler pens and none (that I know of) have cracked. Water buffalo horn, on the other hand, is a different story, and material.
Do a good turn daily!
Don