I'll betcha right now the cause of death is found to be drowning. Having spent many a surface interval doing "little 20' breath-hold dives" with friends for many years without any of us having any incidents (I'm rethinking the wisdom of that with some of the recent articles on possible ill effects), I have my doubts that scuba has anything to do with it. I'd bet that he had difficulty on the bottom getting the float line loose, overstayed his breath, passed out on the way back up, was weighted enough to be negative, sank and drowned. I'd bet it's nothing more complicated than that....The real issue is; did the free dive to a shallow depth (20-25 FSW) shortly after scuba diving, play a role in his untimely death or was something else the cause...
Perhaps the autopsy will reveal something different.
But... even if it does, the fact remains that whatever happened, with no buddy the dive ended on the bottom rather than being dragged ashore on the surface. On the bottom you drown; on the surface maybe not.
Rick