The instructor Gibson failed to maintain the standards she was required to by the training agency, SSI. She had unqualified training assistants, she was not present to oversee the skill her student was performing, and she exceeded the allowable student to instructor ratios (she had too many students, and she was off teaching someone who wasn't even on the course!). The victim may have died anyway: assuming he had been told not to hold his breath and he failed to follow instruction. Whether this is because the unqualified teaching assistants didn't know to tell him that or it was his own error the report doesn't say. But Gibson's failure to stick to the standards would make her "criminally negligent", wouldn't it?
Plus I think common sense and morality should've kept the instructor from doing what she did. What a thing to have to live with on your conscience.