If she had been watching him, she would've seen him, not just heard the crash.
And it have very well happened even if she was standing right where the child was.
What is the point of going shopping if you cannot TAKE YOUR EYES OFF your child? LOL! No sneaking a peek at a price tag!
By 3 years old my children were well enough behaved that they could stand nearby looking at something while I shopped. If they moved out of my sight or were messing with something they had no business messing with, into the cart they went.
Really, I think it is impossible to shop for something (you must look at whatever you are shopping for, unless you have memorized the placement of all the items) and NEVER take your eyes off your child. Even if the child were in a cart, you shouldn't "take your eyes off them" because they try to stand or climb.