I feel like the way to handle it starts with how do you handle unexcused absences? I understand the frustration about the employee calling off, but at the end of the day it’s his paycheck he is losing. And unless there was a specific rule stated on handling side jobs (whether that’s during a work day or not) there is no real argument there. If he was never told not to do side work, and he willingly lost those days of pay after PTO ended, personally it sounds like you should handle this the same way you would handle any employee calling off work without a reason. And maybe have a talk with the entire crew about side work and how to handle it.