I agree that my vellux blankets all have a tendency to wear a little on areas of pressure, like the velvet part rubs off...If she is not asking you to use it due to sentiment, maybe you can explain to her that you want her special quilt to be awesome, and last a long time, and that you are not sure the vellux blanket will be right for it...then offer a different soft backing, as the others suggested...
I also agree how kind of you, and I know you are probably getting just as much out of making it for her..
Blessings,
Kif