First, let me say that it would have been nice if you had said something, but don't beat yourself up over it. I probably would have not said anything either, and as little as I know about sewing, I would have realized that she didn't have enough material.
Now, about the girl who was doing the cutting....she may have been filling in for someone else. I worked at Walmart years ago, and I knew how to use all the devises that make labels, and those lovely price tickets that they attach to fabrics. I can tell you what I really knew about fabrics would have fit in one of the plastic thimbles, and the day the called me and said to go back to fabrics for an hour to cover a lunch....I told them they had to be nuts. I knew NOTHING about working with fabrics. I spent 12 years working as a doctor's tech, could take your blood pressure, check your eyes, make your glasses (actually MAKE THEM), but advise you on fabric? NO WAY.
Fat lot of good that did, because I ended up back there cutting fabric. Hopefully everyone who came in that day knew exactly what they were doing, because I had no clue. However, I do guarantee not one of them got shorted on yardage, lol. Just in case my cutting lacked finesse, I made sure they all got a bit extra.
So maybe next time you hear that kind of conversation, think that maybe the person doing the cutting just got dragged out of the cash office to cover a lunch hour, and butt in...nicely, of course.