The first mistake is assuming that when an employee leaves, he/she isn't leaving in anger. How does the outfit know whether there is anger or not? Also, anger isn't always the only motive an admin might go rogue then up and quit. Mental illness. Drugs. Personal problems. Who knows. They certainly aren't going to announce "hey I'm angry because [enter reason here] and I'm going rogue then quitting".
The data is an asset, just like anything else in the company, and needn't be treated differently just because it's digital.
In my personal experience, mission critical data has always been backed up and kept off-site, usually with the "big cheese" - the person with whom the buck stops. How often is the answer to a simple question: "How much of this can go missing before trouble starts?"
If the answer is "none", the solution is mirroring - real time backups all of the time.
If the answer is "a little but not much", full backups and prescribed intervals with incremental backups filling in the gaps can be considered.
If the answer is "pfffft, doesn't really matter, just not too much", then a manual backup at set intervals would suffice.
The danger here is not the finding of an adequate solution.
The real danger is assuming an employee is/will be leaving on good terms and isn't intent on causing damage.
Assume the worst always, and don your teflon. When Murphy's Law strikes (and it will), you're bullet proof.
FWIW, I have a standing policy - when I accept a notice of termination from an employee with adminstrator[-like] privileges, I say thank you for service and escort them out the door. On the spot. No exceptions.