I hired my guy, because the work I was having done was beyond the fact that he knows how to do the roof work, so that it does not leak. The same reason that when you have a door like the OP, you use the proper materials to complete the job. That means if you have to order a door from the manufacturer to fit the opening, you do it. What you do not do, is place a board at the top of the doorway to make up for an odd size opening.
Only if you know what you are doing, you can make it work, but nine times out of ten, you can notice when someone does a job like this incorrectly. If it was me, I would be ordering a custom sized storm door, not frame out the opening smaller.
The guy has a door alrerady. He's not getting a custom sized door because you think that's the way it should be done.
He asked how he could use it and I told him.
Will it look odd? Maybe, but that's his call. He spends some time putting it in and then he looks at it. If he's happy with the money saving solution, end of story.
Like I said in the beginning, DIY'er's are in it to save money on the job. Sometimes looks are compromised. Not my call. It's when function and/or safety is compromised, I advise other solutions.
Bottom line, it's their house, they can do what they want.
"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."
B. Franklin 1759