I'm really surprised that many posters here think it is illegal to buy or own or use scuba equipment if you are not certified. Where is that written? I'm also surprised that they imply that you do/did something underhand or illegal to obtain this equipment. In my case, I NEVER had to show a card to buy anything, and it was all legal and above board. Some of my stuff was purchased from online scuba dealers. Other I bought new, and the rest I bought at my LDS where I was a student, but the salesperson was not in my class, so I'm not sure if they knew that or not. But the stuff I bought from the LDS wouldn't be considered something you need a cert for, even if you were mistaken that a cert was required (i.e., wetsuit, mask, dive bag, t-shirts, retractors, lights). I now own all my gear, except for a compressor, which will probably be my next major investment.
I took my AOW with a retired Navy Seal, and I'm sure he was a far better diver and could handle unusual circumstances better than the instructors and DMs. Befor he was certified, I would have dove with him, after making sure that we could communicate with hand signals - I suspect that the Navy may used some that are different from recreational divers. He was taking the course so he could dive with his certified wife on vacations.
I am not sure I had to present my C-card to rent equipment overseas, but I did have to send it to the trip organizer, and he may have forwareded it to the dive ops. I have had to turn my card over to the custody of the captain of liveaboards, to be returned when we returned.
I am not aware of any law that requires a skipper to required divers have a c-card, in the USA, since it's not required that they be certified. That said, I suspect it's prudent for them to, as this would help establish a "reasonable" care should anything happen. But, a diver simply not have a C-card would not be cause for criminal charges. Of course, anyone can sue anyone for anything here (USA) so civil charge don't have to be based on any law, regulation, or statue.
I think checking to see if you potential buddy is certified is only going half way. I would also want to know what kind of diving he has done, and how recent. A diver with a C-card that's 20 years old, and they haven't dove in 15 years, and then only in shallow warm water with great visibility isn't really too comforting if I'm planning on a cold, deep, near zero vis dive.