She's just never seen one not toasted. Any diner she's ever been to, if you order a BLT, it's always toasted (true for me, as well).
I never had a "proper" BLT until I left home at 19. At home, a bacon sandwich meant one thing: one piece of white supermarket bread, about two tablespoons of mayonnaise, and whatever amount of bacon you wanted. Load up the bread, fold it over, and that was that. It was never part of a meal, but just as a snack between meals.
To this day, I still get caught up ordering BLTs out - I always forgot to ask them not to toast the bread. I guess I'm lucky in that "toast" in a diner these days amounts to "warmed bread," so I can deal with it.