I would have her measure her mattress, because of the different thicknesses of the mattress its hard to say one size fits all. Basically, the width and length are the same but then there's that drop.
Also, you'll want to find out how she's planning on displaying on the bed. If she's someone that will tuck up under the pillows then you need to add some length. If going to use with pillows, still different.
Find out her mattress thickness and be sure to add to all sides and how she's intending to displaying it, then adjusting the pattern accordingly.