Bodies will not float back up if lost in extremely deep water, such as below 300 feet. The gas expansion in the body will not increase enough to change the bouyancy.
As for saving someones butt, I have snatched a few divers below 200 feet, one at 240 on a Cayman wall and another at 278 in a deep sink in the Gulf of Mexico and assisted them back up to where they could think properly. But that was back in the deep air days.
I had one guy, that was so far out of it at only 184 feet, I had to take him by the shoulder strap and assist him to do all his decompression stops all the way to the surface.
Many years ago I had a guy pass out at 154 feet, spit out his regulator, so I brought him back to the surface. Luckly, we had just got to 154, so I did not have any decompression to do yet. He woke up once he hit the surface. He turned out to be OK, amazing I thought.
Most divers react different to deep diving, so its hard to tell just what will happen with a new deep diver.
I would suggest, if you are going to try a deep dive, sub 150', I would take someone who has been there many many times to watch your butt for you.