Tell the other employees that they aren't privy to their coworker's personal situation and that you are at liberty to allow any employee to not show up a few da

Tell the other employees that they aren't privy to their coworker's personal situation and that you are at liberty to allow any employee to not show up a few days a week, if you believe doing so is essential to their physical or mental health, if it's not hurting the bottom line, and if it's not a quid-pro-quo situation. If the other employees want more time off, the correct solution is not to ask them to pull all nighters but just ask why. Usually extra time off can be accomodated with occasional work from home or allowing shifted start times. Or, possibly, your employees didn't even want the two days off a week -- maybe they were just feeling left behind and unappreciated that you just loved that coworker so much, and they came to perceive his odd working hours as a status symbol and reward.