S3 black screen bootloader corrupt?

[Bosgarage57]

Hey guys, I have a friend that isn't very techy. He said his S3 just went black screen, took it to a repair shop and said that the bootloader is corrupt, no possible fix. Do you all know of anyway to fix this? It hasn't been dropped or in water. He isn't rooted. It is an unlocked S3 (AT&T).