It was stated in an email(contractual agreement) that referrals would earn you 10k. That agreement was met during his employment. The company should have given it to him upon completion. They failed to do so. He doesn't forfeit whats his upon departure.

It would be like you being offered an additional 10k to do something for a client/boss. You did it, shortly thereafter left but hadn't yet received your compensation. You'd be within your rights to go after them via the labour board/a lawyer. If you don't see that, I don't want to know how you treat those around you.