Earlier quoted context omitted.
Many employment contracts expressly forbid you from accepting other employment without prior authorization from the company. If his employment contracts contain that provision, then he would definitely be in breach. But... damages would probably be limited to just the salary that the company paid.
Definitely true for most high-paying companies (meaning experienced enough to not have employment contract full of beginner's holes like this). Explicitly forbidding another full time employment, full rights on all intellectual property, non-competing agreements etc. Also explicitly expecting to work ie 40 hours. I would take this with grain of salt the size of Jupiter, since such behavior would quickly show on any e…
Personally, I haven't had an employment contract for over 6 years now, and in fact to stay in the clear one should ensure they have at least two simultaneous clients - though most ignore that, at least in Poland.