It could also be where she has it located...
There's things like "micro~climates"
If they are close to the base of a house, sidewalk, cement patio, brick... etc..
Something that can keep/hold heat...
I am in
growing zone 5, and I can over winter a dahlia outside, at the base of my house, it's by a stone wall from the basement, it's in a raised bed, also has a cement silo brick walkway next to it, *two hot water tanks and a furnace in the basement*, plus I live in town, with lot's of sidewalks and paved roads...and large
trees...
Many factors can play a role with
plants and their demise or growth or ability to grow it at all!!!