"He" is me. While it is true that every cigarette marginally increases the risk of cancer, injecting the smoke of two cigarettes into the anus of a girl who already smokes isn't effectively going to increase her chances of getting cancer. All fetish play involves some risk and this risk is at the lower end of the scale.
I can't imagine that this would much different from them being introduced into her lungs. I think you'd be hard pressed to find anyone who ever got cancer from a single cigarette, no matter how efficient the uptake of its carcinogens.