I can understand what you mean about the scuba gear... kinda spooky how it happened, but it's perfectly good stuff
I'm Divemaster cert'd, used to work at a Dive shop... you got a good deal!!!
As far as the climbing gear...
As long as it passes a thorough examination, I'd think it should be just fine.
Not knowing the history of the biners and such I'd give them a really good going over. Might personally just use them for rigging and still pick up my own biners for personal safety.
Don't see any problems with using the harness/spikes/etc though.
What all gear is included and how did he pass on??