100% approved by our army of gals and our field report earns us a few bucks.
Pass on the zipper. Putting a zipper on a silk pillowcase seems to be a popular idea, but there are two things wrong with that. (1) A silk pillowcase is supposed to be smooth, so who would want a hard zipper running down the side? (2) Those pillowcases tend to tear apart at the seams in the wahsing machine after a while. You want to get your gal the "envelope closure" pillowcase, like at the link above. (You might even contact them to verify this, as companies sometimes change things up over time.) This is the open-sided style that you see on just about every pillow on Earth.
Thnk about getting her two pillowcases. One is good, but lots of gals like to lay on their side and put an arm around another pillow. Plus, when she makes the bed, a matching set of pillowcases will balance out the look of it all.
You don't need to give her a pillow with this gift. Each gal has her own pillow preferences...stiffness, thickness, texture, and so on. Any pillow you pick out is likely to be wrong. Just stick with the pillowcase.