Answer:
Jill omitted a factor pair , which affected the GCF and factored expression.
Explanation:
Jill's expression
13xy-52y
Factor of 13xy: 1,13,x,y
Factore of 52y: 1,2,26,52,13,4,y
But Jill give factor of 52y: 1,2,26,52,y
Jill make a mistake when Jill factorise 52y .
He does not take 13 and 4 in his factor.
GCF :


The common factor of both terms are 13 and y
Therefore, GCF:

But Jill give GCF:y
Jill give factored expression : y(13x-52)
But actual factored expression: 13y(x-4)
Therefore , Jill omitted a factor pair which affect the GCF and factored expression .