If the only drive in the system is the RAID drive, you will have an interesting problem. If the RAID is only for data, you can backup what is on it, scratch it, build a new RAID 1, and restore the backup. If the OS is on it, you will have to do a re-install, and the Dell restore disks may or may not choke when restoring to a slightly different configuration.
We would love to walk you through options and the process, as long as you are willing to accept the risk of a complete loss of everything on the system and the need to run the factory re-install disks. You do have those disks, do you not?
EDIT: Or run it as-is and do frequent backups, so that if the RAID0 fails your father doesn't lose much.
Call the salesman who assured you that it can be easily un-Raided and have him/her provide instructions!