That's a very good point. I was kind of dancing around that when giving the reasons to use a pre-hung door. It makes it easy to get a perfect door that is square, closes properly, etc. I also said that he is underestimating the effort in building a door and HANGING it. The above though really nails it. It's a 50 year old framing job. Has he even put a square and level to it?
That's about where I'm at too. I just had visions of him spending a lot of $$ and time, screwing around, building a door. Then he goes to hang it and finds out the framing is out of square. Then he spends hours trying to make it fit that opening. Or that it warps 6 months later. I'm not a big fan of re-inventing the wheel when suitable solutions are readily available.
Best advice I can give at this point is either fix the old door or look at videos of how doors are installed.