For a down and dirty answer.... (but there is always 2 sides to every case.)
I would have to say per the hypothetical of the OP, most likely, Diver 1 will not be criminally charged with manslaughter (or any other crime) for the death of Diver 2 in that he did not do anything wrong, criminally anyway. Even though Diver 1 strongly urged Diver 2 to go on the follow up dive, he did not recklessly perform an action which is likely to cause death or great bodily harm to Diver 2. Diver 2 was still responsible for himself and knew better than to dive again. Diver 1 was not acting in the capacity of an instructor so he cant be criminally responsible.
Now in civil court it is a different matter. You can sue anybody for anything. Family member of Diver 2 would easily find a very sue happy personal injury lawyer to go after Diver 1, the person who administered O2, the Boat (if any), the company/person who owned the dive site (if any), the organization who certified diver 1 and 2, and any other person/organization who was any way affiliated with this diver and the situation. They would probably pick all kinds of claims, like breach of fiduciary duty on the part of Diver 1, failure to provide adequate medical care to the person who gave him O2, improper training or certification for the organization who certified the divers, unsafe dive environment by having the site open to divers anyway, and any other type of claim that they could make against anybody else. That is the Key! Lawyers that think they have great cases will throw the entire book at a defendant and see what happens.
Now, I think that this sort of case would either be settled out of court (as 90% of all cases are) or would be kicked out as Diver 1 and all the rest cannot keep someone from doing harm to themselves and we are all trained, not to go beyond our training AND if something happens, abort the dive and stay out of the water afterward. There is no duty to others in this case. Diver 1 was not acting as an instructor for Diver 2. Diver 2 choose to dive and he knew better. Just like the friend who knows that you are drinking and lets you drive your car or even encourages you to drive your car, is not responsible for the DUI or the injurys you cause because you drive drunk. Just can't happen that way, even if your friend was a cop or a drivers ed instructor.
The bigger problem comes from the defense lawyer hired to defend Diver 1 in this case charges $400 an hour plus costs and Diver 1 has to mortgage his house to pay for this case because the case drags on for so long because of all the tactics of sue happy personal injury lawyer. He is so distraught over situation that he can no longer work as an instructor, even though he is not admitting fault for the death of Diver 2. Since he cant work, his family cant afford the mortgage payment for the lawyers fees and they lose their house in foreclosure. Now he is on food stamps and Prozac and cant dive.
Now as for the McDonalds case that was addressed, you have to keep in mind lawyers need money too! So here is a lawyer who got a call by a lady who says I got burned while at McDonalds. Lawyer instantly perks up and hears old lady, burn, McDonald'$$$$$$$$$. He finds that McD serves coffee hotter than any other company and does not label their cups for hot coffee. Lawyer's light bulb turns on and says, I beat I can get a decent settlement out of McD'$$$$ cause they hate this sort of publicity. McD says you are nuts so go ahead and sue us. But what McD forgot is that this was a civil case and 12 jurors get to decide what happens. And 12 jurors see an old lady with a burned crotch from McD'$$$$ who has lots of money and old lady does not. So they give her a big award for lots of money. Lawyer says hallelujah, my student loans are paid off, my kids can go to ivy league and McD's was stupid for not settling cuz we would have taken $50,000.
Just my $400 an hour's worth. I expect prompt payment.