Think the circumstances dictate a lot. If it’s clear the employees intent was to burn bridges, then it’s on them. But it doesn’t sound like they were trying to crap on you or you wouldn’t be feeling bad about it now.

I think that there are certain fields where, once you know an employee is leaving they let the employee leave immediately (in instances where competition concerns are at issue). On the other hand, many service industries will ask employees to give 2 weeks so they can find a replacement, but the only real incentive to do that is to keep you as a reference or simply to do you or your company a favor. They don’t owe it to you. You paid them a paycheck. They provided services. It was mutually agreed that they were at will and could quit at any time. If an attorney wrote the offer letter and or employee handbook, it probably says that in writing somewhere.