If anybody has a solution, I'm all ears. Kinda need one soon.
E: Found the problem. It was an extra key-value pair in the map properties that pointed to an extra wad for some ridiculous reason. So obscure, no wonder there was no results in google.
I've tried googling but nothing related to this issue pops up.
I tried compiling one of the maps with the old .exe's, removing any custom commands, and it still causes hl.exe to hang and close with no error popups. He can run it in GL, but his current rig is far too crappy to do so. I understand it's 2018 but this is a problem I'd like to have a solution for, in the stead of telling the player to upgrade.