She can't say that she likes the idea of lighting her friend on fire, even if it won't permanently damage her. Fortescue's dual-fired like this before, except it was with another warlock, Mattli, who was undamaged by fire and could create his own.

"Are you certain you want to burn just for some vines?" Fortescue can't help but wonder. "Why don't we move in and see if we can't do some pruning, and if they go for you I'll light you up to dissuade them?"

Ashley may not be hurt, but it's the principle of the thing.