RAID only protects against single drive failure - depending on how the array is implemented, it doesn't protyect against controller failure, or a power spike taking out both drives, or memory failure causing file corruption (or accidental deletion of a large file) - which is why I suggest it isn't a backup solution. However it all depends upon the amount of risk he is prepared to accept. You are right about DVD/CD which are not regarded as a long term archival media.
If he is storing files in a raw (uncompressed) format then each file will be of the order of 20Mb each - or 50/Gbyte. You don't say if he is doing this commercially or not - if he is, he will be storing various copies in different formats, so he will soon use up that 500Gb, so he will need some form of archive. (Wedding photographers - for example - often store photographs for some time in case the client wants more prints, or perhaps different prints at a later date. Some portrait photographers undertake to keep digital copies of prints clients have ordered for many years.)
If he is a hobby photographer then his storage requirement may be less initially - but still something he wiill need to think about sooner or later.