By that letter, if you first assemble the lower as a pistol you're okay. If you then assemble the lower as a rifle it then becomes a rifle and cannot be made back into a pistol.
When I built my first AR I had my stripped lower registered as a rifle for two reasons.
1) I was only 20 and couldn't own a pistol in Illinois
2) I planned to build an AR pistol as soon as I turned 21
When I built my AR pistol I bought another stripped lower and registered it as a pistol so I would never have to hassel with is it a rifle or pistol. I now have a pistol lower I can build into whatever pistol configuration I want, and a rifle lower I can configure how I want. Now all I have to do is build different uppers and attach them to their perspective lower.
If you friend never plans on building an AR pistol just register it as a rifle and he'll be fine. If he wants to build an AR pistol register it as a pistol. Yes, you can register it as a frame or other, but it's always easier to choose one path and run with it. If he wants to build another AR later just buy a new stripped lower and register it as which ever setup he wants to build. If the cost of another lower is the make or break cost of a build you probably shouldn't be doing the build.
I'm not trying to step on anyone's toes by saying all of this, just trying to make things easier for someone trying to follow the stupid laws of this dumbass state in which we live.