Relocation stipends are pretty much always paid contingent on signing an agreement that if your employment ceases within X timeframe (no matter the cause) then you have to pay it back, or at least part of it. If an employee has a problem agreeing to such a provision, then they simply don't get the stipend and they have to relocate on their own dime.

If you owe any money would depend on the exact wording of whatever agreement was signed. It probably says "for any reason."