Join Date:Joined: Feb 2014
He most likely corrupt his EFS partition.
If you have no back-ups of the EFS partition, you in a whole new level of trouble.
Try to a cache/data wipe in the custom recovery. If that doesn't work, flash back original ROM with ODIN. If that still doesn't work, you going need to get IMEI back with one of the service boxes that support S4 mini.