Not nearly as serious as your BiL but I had a discectomy nearly thirty years ago. It was early days then for that op and meant hooking my spinal cord our of the way. I was told 20% chance of complete success, 20% chance of the wheelchair and paralasis and 60% chance of staying same. I prayed for anything but staying the same. Your BiL is in a desperate position and it's terrible for all of you, especially him. He must have got to the point where anything is better than how he is now, but the odds of death must be very low or they wouldn't risk operating.
I do hope he's as lucky as I was and all goes well. 