So, you have your watch folder -- I'd just have that on an internal drive and not have it shared. From there you can do the star/color/rating.
You can turn LR preferences to automatically write the metadata to a file so that would save you a step -- if I remember correctly it only affects files that are being changed.
Once you've done that all you have to do is within Lightroom move the images to the shared drive/folder. Your assistant would need to initially import them (it should automatically recognize the xmp sidecar files and apply them). From there I believe that your assistant can bring in more files as you add them by right-clicking on the parent folder and choosing Synchronize.
But, having tossed all that out, I've never actually done anything like this and you will want to test/verify/experiment with things until you have what you know is a working system. Don't just take our word for it!