Not necessarily, as a faulty motherboard can cause issues that seem like it's the drive at fault. I've tested the install Boxed version on multiple computers with different operating systems and different drives, including a USB 2TB HDD, all with varied results as in "Install time", "disk read time" and "stall" time, the only consistent variable was the 2 errors I had which were to do with 2 files not being installed and it was the same two files every time no matter the OS or hardware Configuration weather it was installed from USB DVD Drive or USB HDD or weather it was installed from C: drive on different SSD/M.2.
All I'm saying is, is that he could be spending time enjoying FSX instead of pulling his hair out trying to figure out a ghost problem which could be anything at this stage, it may not necessarily be his drive at all and that it could be a faulty SATA port on his motherboard causing his problem, I think it has something to do with windows 10 and how it handles the format/install of the FSX DVD's. I turned off all Virus protection when I did the testing also.
By random chance one install went as it should but not without taking a long time and that was from the USB drive.
I would suggest putting your motherboard back to default and test again, then switch SATA ports and see if that makes a difference.
Dovetail did make quite a few changes to the Steam Edition so it runs on the new O.S Systems without problems and I have had zero problems at all running this version. To me it makes logical sense to purchase this version as Dovetail already made the changes necessary for it to run on Windows 10, obviously it still has it's problems but not installation problems.
More to do with people not keeping windows up to date so redistributables are not installed hence causing launch problems.
As you might also know we will be having to make sure we have the new 1909 Build to be able to run the New MS FS2020.
So this is a must anyway if he was to switch to the new FS2020.
Just some more info from what I've experienced so far with this problem.