Hopefully you have been keeping the GM updated on these issues-so that he/she is not blindsided by them. You need to talk to the GM and make it clear what J wants and how you have tried to deal with it. I was a manager for MANY years, doing schedules, since I was the nurse in charge of a 64 bed unit. Someone will ALWAYS be unhappy. All you can do is your best to accommodate their requests, which, should be in writing and submitted per company policy, unless it is an emergency. If the employee has something come up-then THEY are responsible for switching with someone to cover that shift. Period. The company/patients cannot suffer. In fact, in nursing, you can lose your license if you leave without proper coverage. It is called abandonment.
Document as you have been advised.....altho, I will tell you I took one employee into a stairwell one day-no witnesses.....said what I needed to, and clearly told them I never said this.......lol and they never questioned it and straightened up. They knew it was my word against theirs, and they had been written up numerous times, so had one foot on a banana peel.