His joke is that fluoride can be used to make extremely dangerous substances
From the wiki on the one he mentioned:
This oxidizing power, surpassing that of oxygen, causes ClF3 to react vigorously with many other materials often thought of as incombustible and refractory. It ignites sand, asbestos, glass, and even ashes of substances that have already burned in oxygen.
I think the crux of it will be, would she respect OP's refusal to have that kind of power over her, and would she expect some sort of reciprocal act for some MAD outcome.
I'd wonder where she got ideas like that and find out if it's an integral part of her life.