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

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

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.