Playing the devil's advocate here.
Duty to act: As a dive buddy do you have a
legal duty to act? Now as a paramedic on an emergency call, I do have a legal duty to act. As a dive buddy, do you? I won't argue a moral and ethical duty to act, but is it a legal duty to act?
Solve your own nightmare: Buddy wants to enter the wreck? Get in front of him and do the "No F'in Way" sign. If he still enters, you made the attempt to stop him and he didn't listen. I will way without the correct training, I ain't going after him. I will try my best to keep him in sight and maybe shine my light in the entrance, but not going to put myself in unneccessary danger due to his stupid act.