AMD Crossfire is possible on just about any motherboard with two PCIe slots. Only those boards that are specifically SLI certified can do Nvidia SLI. Your motherboard supports Crossfire, but not SLI. Sorry.
you won't be able to crossfire either even if you had AMD cards. The 2 things are the same and basically require the same things.
He would be able to Crossfire as that board is Xfire capable.
Gigabyte GA-H55M-USB3 comes with two x16 PCI Express 2.0 slots and two standard PCI slots. The second PCI Express x16 slot works at x4 and can also be used for installing x1 video cards. These two x16 slots support CrossFire configuration, but not SLI.