But it depends entirely on (1) the exact terms of the agreement and which state’s law governs the agreement and (2) the attitude of the people running the company for whom you work.
I will throw out a possible third and fourth factor: WHY the employee is leaving and the budget.
I worked for a company that had this type of relocation pay back clause in their agreements. We had one employee that had to leave because a parent developed serious medical problems and needed the employee for long term care. The employer did not ask for the repayment under the agreement. However, the guy they hired that irritated the CRAP out of everyone he worked for and with was made to repay the full amount under the agreement when he left after 6 months.
If the money would be needed for the possible relocation package of the new hire to replace the departing employee then repayment would likely be sought. If the budget is not strapped or there is no intention to replace the employee leaving then they may not seek it.
Too many factors to give a one size fits all answer on what they will do.
"That's just my opinion, then again I might be wrong." Dennis Miller