The diagnostic device he has that gives the error codes won’t help for this problem.
He’s going to need, and know how to use, a multimeter, a schematic, a wiring diagram, and perhaps a battery tester.
The code reader he has only reads error codes stored in the engine’s computer. Those codes only store if the car’s sensors detect an error on something that may cause excessive emissions if not corrected and errors in mandated safety systems. Contrary to the belief of the general public, they do not and are not designed to monitor all the operations of the car’s systems. And they’re definitely not designed to detect electrical problems unless they fit one of the above categories.
There is one outside possibility to trouble shoot this. It sounds like you may have what’s called a “parasitic drain”, something drawing juice from the battery while the car sits. Try removing all the unnecessary fuses (for accessory systems) and see if the drain stops. If it does, replace them one by one. When a specific fuse replacement causes the drain to return, you’ll know which circuit is the problem.
Schematics and wiring diagrams can be obtained free of charge at the dealer’s parts counter once you know what circuit you need. From there, your BF can look for a possible short to ground, perhaps due to a chafed insulation spot. It’ll probably be somewhere where the wiring harness (bundle) passes through a body cavity, like from the body to the door.