I know someone planning something dangerous, what should i do?

I know someone thats going to use a flame retardant suit to light themselves on fire for a few seconds. Say I were to go to this event and offer to man a fire extinguisher or help out in any way, or just watch. So if i did this, and the cops showed up or something, would I be getting in trouble? (this event will be occuring at a private residence) Also, what do YOU think I should do? (The person doing this has an extensive knowledge at the forces at work here, and appropriate safety precautions are being taken.)