MacBook Pro 15" A1398 black screen after battery replacement

Hi everyone, a friend asked for help with his MacBook that was stuck in a boot loop, displaying only the Apple logo and a loading bar. After trying SMC and NVRAM resets, I reached the user menu, but the loop resumed after entering the password.

I noticed a bloated battery before opening the device, and inside, I found a very defective battery. Disconnecting it didn't stop the loop. I found a video suggesting disconnecting the display and trackpad, which worked—albeit slowly—using external peripherals.

Research indicated the boot loop could be due to the battery design. I ordered a new battery, backed up the data using the same workaround, and cleaned the dusty internals. With the new battery installed, I only heard the fans without any display. An SMC and NVRAM reset brought back the boot chime, but still no image.

Shining a light revealed an image without a backlight. External monitors worked fine, suggesting a backlight fuse issue. Despite checking continuity with a multimeter and cleaning connections, the issue persists.

Any ideas on what might be causing this? I've handled similar Apple device repairs but never encountered this problem before.