Build a backup contact. A referral submitted before someone leaves may still enter the system, but you should not rely on an uncertain internal process you cannot verify.

The more durable value of networking is having people who know your candidacy, not only one referral transaction.

Thank the person, ask them to submit the referral while they still can if appropriate, and continue building relationships in the target firm.

Do not pressure the departing employee for guarantees. Treat the referral as helpful, then keep your application strong on its own. For a related reference, see When Should You Ask for a Consulting Referral?.

I would not overthink the etiquette side of it. Clear and professional usually works better than trying to find a perfect script.