RAID is not a backup. It's a live duplicate. If something (virus/hacker) attacks the server then BOTH drives on the RAID will get damaged. If the RAID controller screws up then BOTH drives on the RAID will get screwed as well. Doubt me? Go to webhostingtalk.com and look at the little web hosts who cannot recover from a failure due to BOTH RAID drives getting hit with whatever killed the server, but they didn't have a true backup.
Get the customer a much bigger hard drive and a backup program that works for their environment. DO NOT just make one backup, you need to do several, even if they just go on the same backup drive. DO NOT just backup, do selective restores and if possible a complete restore to a bare drive if you do not already know the software. Nothing worse than having to figure out a backup programs restore process on the day of a drive failure (happens thousands of times every day around the world).
I've seen countless RAID configs fail and nobody on-site ever knows how to recover. Worse if it's the controller that fails you are f-cked in recovery. Also, just making one backup that gets overwritten daily or weekly will most certainly fail when the client needs them the most. Likewise if a customer calls you and says "We need a folder restored from last week" and your backups are daily only then you are screwed.
Backups are the most important thing in a computer and very few people realize this until they are looking at a dead machine with the life of their company hanging on what should be a simple restore.