Well, corian is as safe to work as anything else that generates fine dust. If you are enlarging the hole, I would suggest that you take the template that comes with the new sink and transfer it over to some 1/4" ply, Hardboard or even heavy cardboard. Use that to lay out where you want everything and then use it with a router and guide collar to do the actual cutout. Corian will get sticky and eventually melt, so keep the heat down, nice new sharp down spiral cutter and light pressure. The lady might not like having a drop in sink with a lip after being used to a built in rimless model though, you might look into an undermount SS model. You can use a standard round over bit to smooth out the hole after you cut to size, then mount the sink up from underneath, that way she will still be able to wipe water and whatnot into the sink without a lip in the way.
Then you can make pens out of the old sink. Corian will flatten out quite nicely in the oven, just verify what temp and max time online somewhere. Rough cut the curved parts, lay in a flat pan and heat till they get flat. Just be sure you have good ventilation, as in NOT IN THE KITCHEN. a toaster oven on the porch or with the garage door open and a fan is ok.
