I'm not sure I agree with ultimatum's, as normally with a deal breaker I'd just walk away, but I feel that I would have to issue one in this case, due to safety.
If your partner said they were going to do something that would 50/50 cause him to die and you'd already taked to them about how you felt/encouraged them not to do but they were still considering it.
It's an activity, that's all I'm saying.