Your problem isn’t that this employee got another job, it’s if their performance and work suffers for it. if they somehow are able to do both then your company is lucky they have an incredible employee and should be liking for means to retain them
You are technically correct, but I would posit “secretly holding two full time, demanding, salaried jobs” is an extremely strong predictor for failing to perform sufficiently at either. Given that, it’s a simple matter of weighing those odds against the costs involved in proving it.
One common argument is if you can’t tell their performance is impacted, they must actually be performing adequately. Except, in a different context, we will happily explain how our own performance can’t be measured by number of lines changed or bugs fixed.
