that's terrible. We all have to start somewhere. I've been fortunate to have had good instructors so far. And the way I learned to sew a "perfect" 1/4" wasn't until I took a class where we were making a "Plates around the World" quilt (I think this was my instructor's own pattern). Each plate had 8 angles going together (similar to dresden plate except there's no hole in the middle). If you didn't sew it exactly 1/4", you either had a gap in your plate when sewn together or you ended up with a hat LOL It took us about 4 months before most of us got it just right. It was a great teaching technique.
I hope she doesn't get discouraged. Just tell her to hang on and practice. I never worry about exact measurements of a pattern anyway. I figure as long as I make the same change (or booboo) throughout the quilt, it still goes together.
I did a small quilt one time that required 3-7/8" squares for HST. I didn't want to deal with that measurement so just used 4" squares. My quilt looked just fine.
Have your sister-in-law to ask the teacher if she has her "Quilt Police" badge LOL Maybe that will calm her down a little.
Good Luck to her !!!d