I reframed a doggy portrait pastel last year which was done on sandpaper. It was 70 years old and had been sandwiched between sheets of glass all that time. Apart from a slight blurring, it was no the worse for this. Although the top sheet had a nice ghost doggy image on it.
As for fixative, not a problem if the artist uses it, but the correct technique is to fix lightly in stages. Do a bit - fix it. Do a bit more. Trying to fix the whole issue at the end is not the way to do it. No way should a framer be persuaded to fix pastels.