Also, she is absolutely positively only going to one store.
I think that's part of this that I just don't connect with is the refusal to rotate. I will definitely and gladly shop at multiple places. There are some people where it's just off the table. Maybe it's how much you hate the act of grocery shopping, but maybe it's also the size of the family and the different types of stuff that you need.
How much would a single person save by shopping at multiple stores vs. paying a little extra by buying things that aren't on sale at the one store, but might be on sale at another store?
How much extra time does it take to go to multiple stores?
If going to multiple stores saves you $10 on your weekly grocery bill, but takes an extra hour and $1.50 in gas. It just isn't worth it.
OTOH if you are family of 8 and going to multiple stores saves $80 on your weekly grocery bill than it is worth it.