How about this one reason that should be enough to hit this home?
A user with enough power to do, but not enough knowledge to do, has the ability to destroy all of the data you hold dear. Downtime and data loss are already a concern when you have people who are trained to do this, so imagine what someone with no training could do with those tools?
Security, data protection, business resiliency, uptime. All are valid reasons not to. I thought this stuff was common sense and that's why they hire IT people. Conceivably, this manager could have anyone he finds on the street manage his storage...
I would figure out what they are trying to accomplish though, because it's quite odd that a user would ask for admin access to the netapp storage system. Maybe they are just asking for administrative access on the NTFS file system.