zmprov modifyAccount realuser@domain.com +zimbraMailCatchAllAddress @otherdomain.com
 

Do they have any admin permissions at all? (zimbra admin console gui) You could use soap:

Domain Catchall:
If you want to set up an account „user@domain.com“ to catch any mail not delivered to existing users in the domain „domain.com“, you can configure the account as a domain catchall.

If the users „john@domain.com“, „webmaster@domain.com“, and „xyznobody@domain.com“ don’t exist, and mail arrives for them, it will be delivered to the catchall account „user@domain.com“. This will increase the amount of spam delivered, and can lead to being blacklisted. To remove the catchall from an email account, unset the catchall address: