Unfortunately, that site has been fubard for a a while, again (only the main page loads). Or is it just me?
Is there a specific reason you need Turbo C 1.0 not the freely available Turbo C 2.01? Turbo C 1.0 was very buggy, not sure if buggier than MS C 6 but neither was exactly a shining beacon of quality code. Version 1.5 was much improved and 2.01 is reasonably solid.
Unless he's stuck on C, I'd recommend Turbo Pascal 3.01A for DOS.
IThough I'd suggest running a more recent version on a modern machine in DOSBox, then copying the executable over when done instead of programming on that in the first place.
You might try winworldpc
http://winworldpc.com
They've got Turbo C 1.5 and 2.01, as well as a few others (like Microsoft) you might try.
Is that plus the stock 256k or the loaded out 512k? If the former it really might not have enough 'room' to run a whole lot of... well... anything.
Normally, I would suggest trying early QuickC for this but I don't know if the HP Portable Plus has the ability to run the debugger. When writing software for an old system, I usually write on a new system and test inside a virtual machine. That gets rid of 90%+ of the bugs before I transfer the code to the older system. Especially with an unconventional system, it helps to have a debugger on the actual hardware to catch any surprises.
Huh. I had never heard of that HP. It sounds interesting...and slightly quirky.
Would Small C work?
Somewhat OT (or premature), but HP had a "ROM IMAGE Development Package" for this device so that it could be used as a development system for applications stored in plug-in ROM modules (the device's ROM drawers).
The manual is here: http://www.jeffcalc.hp41.eu/hpplus/files/vger.pdf
Summary of commands, etc.: http://www.jeffcalc.hp41.eu/hpplus/files/makerom.pdf
Software: http://www.jeffcalc.hp41.eu/hpplus/files/tools.zip
Other useful info is at the same site, including the tech ref manual: http://www.jeffcalc.hp41.eu/hpplus/index.html
For misc software including a "Programmers Tool Kit" (don't know what's in this): http://www.hpmuseum.net/exhibit.php?swc=11