I haven't sewn with it but I would pre-wash the OTHER fabric but not the minky. I would cut the front and back the same size, but I would select the other fabric very carefully. Read the hints here:

Here is a link that might be useful: Minky Hints

I made a two throw blankets with polar fleece on one side and cotton weave on the other. I made them two different ways. One I used a whole piece of cotton fabric, on the other I sewed together four pieces together horizontally. I stitched across horizontally on both sewing both sides together, the one with multiple cotton pieces is more "stable". Oh... I just realized you are making a duvet cover...Maybe sew some backing (inexpensive white fabric or even a bed sheet) to the wrong side of the to keep the minky from stretching and sew across or up and down to make it more "stable". Pin it together and sew from the middle of the fabric to the out side so you don't get too much gathering/bunching up..Preshink everything. A walking foot would probably help, I've never used one though.

Another thought, maybe have her come shopping for fabric with you, maybe she will then understand the difficulty and be able to find a more suitable fabric that she would like even better. Sometimes people who don't sew just don't understand why things won't work well.