You are conflating different issues. The only relevant issue here is that he fulfilled his end of an agreement: to refer an employee who completes 6 months at the company. That is done, and he is owed the referral fee/bonus. Whether he left the company after he did the referral is immaterial. By default the debt the company owes him doesn't simply disappear.

imho you should revisit the term: 'referral' bonus