I figured that the "windows back up" drive usage pattern will be more similar to cctv usage than a NAS RAID, which is why i thought purple... May be i am wrong.Well, the purple isn't made for that, is WD's line.
It has firmware that is tweaked for video performance...
Could you? Sure.
Should you? That is your call.
For simple backups any drive could be used. I'd go with something like WD blue or similar drive. If they have data that is critically important to them they should also consider some type of off-site backup system or cloud based backup.
~snip~