The HP Technical Exceptions Matrix indicates that 6.1 is the minimum supported RHEL 6.x version for BL460c Gen8:
http://h18004.www1.hp.com/products/servers/linux/supportmatrix/rhel/exceptions/rhel-exceptions.html
The fact that your customer got a "no disks found" error message tells me that the Smart Array controller driver (the "hpsa" kernel module) in RHEL 6.0 is too old to support the controller model used in BL460c Gen8. The necessary update was included in 6.1.
The Proliant Gen8 servers were released on February 13, 2012:
http://www8.hp.com/us/en/hp-news/press-release.html?id=1169355
RHEL 6.1 was released before that, in May 2011. In effect, HP allowed the necessary Smart Array driver updates to be included in RHEL release when the actual server model was still in prototype/testing stage, so that the users won't have to deal with extra drivers on OS installation.
There may be other updates in RHEL 6.1 that are important to BL460c Gen8, too: for example, the CPU model and/or ACPI feature support, or the support for the integrated Matrox G200 display controller. As a result, backporting the storage driver for RHEL 6.0 was probably not considered useful.
If you're interested, see the RHEL 6.1 Release Notes:
and compare the list of updated drivers & new features to the BL460c Gen8 specifications: