Wow, that's amazing. Perhaps instead of a USB-C cable, we should just buy or lend him a second Switch. You can find a used Switch (tablet only) for around $90 on eBay.
Puts to rest any thoughts about whether the USB-C splitter could affect the battery life of the injector.
I think the reasoning for a splitter is so that even from full charge the DI might outlast the console, but because it only has one port, the splitter would be required to keep the switch charged and to inject