The only thing I can think of is that there was some sort of event where other people were being willingly set on fire. Like if were some sort of performance that went wrong, I could see there being a reasonable defense that the performer didn't know she didn't want to participate.

Most of the time when weird questions like that involving consent come up, I assume it probably involved some kind of sex thing.

my lay person understanding of the law is that in situations like these you don't need to prove what should be so overwhelmingly self-evident

there's probably some Latin phrase for it a lawyer would just know but when a thing is so far away from a gray area of reasonableness you're fine

you shouldn't need hard evidence, people as a rule don't like to set themselves on fire