God darn! It all depends on your backup strategy. If you have offsite backups you might be able to restore from the recent offsite backups. The restore is not fun I might add.
You will need to create a separate AD domain (maybe in a VLAN) matching current AD Domain, Exchange Administrative Group by installing Exchange App (MSX Server name must be same as current Server in question). Re-create all the AD domain users. Then restore EVERYONEs mailbox (the mailbox store) on the Server. Then you can reconnect to the mailbox with the username in question.
Again, do this in a separate LAN.