During a raid rebuild, it should be possible to access not only the data, but also the status page in the web interface, which will indicate that a rebuild is in progress and also provide an estimate as to how long it will take - since he is unable to "access the NAS in any way", I would think there is a problem of some sort.
Unfortunately, since he mentions being unable to access the NAS via ssh, something which is not supported in the standard firmware, I would suspect the NAS has been hacked in some form, and at that point, any suggestions as to the cause become little more than a SWAG.