The difference between LR and PS is that PS will destructively edit the image while LR will leave the Original unchanged. I don't think you want to destroy the master using Photoshop. In that case, what you will need to do is create a derivative much as you would do if you were using {Edit-In} in LR. To accomplish this, you can implement a regular Export. In the "Export Location" section of the Export dialog, Check the box labeled "Add to this Catalog". Once Exported, transfer the derivative image files from the Export Location designated in the Export dialog to the assistant's computer. When the assistant is finished, transfer these edited files back to the location designated in the Export dialog and LR will find them where you created and cataloged them (As long as there have been no name changes).
Lightroom will maintain previews from the original export and will not immediately reflect the changes in the previews until you force LR to refresh its Preview cache.