There are so many things that come into play. Has she used one before? What does she anticipate cutting on it? How much space does she have? Will she be cutting yardage or precuts?
I have lots of cutting mats that I use for different purposes. One is 5 feet by 3 feet or so, then I have 24x26. 18x24, two rotating ones and a couple oddball ones. Brand does not make any difference to me, I am more concerned about function.
I sew clothes and use the largest one for that. I use all the smaller ones for quilting.
Does she have rulers? What about a rotary cutter?