Mac can't detect usb 3.0 harddrive
 

My sister has a Macbook Pro running OS 10.7.4. She tried connecting her friend's harddrive, a 500GB Western Digital My Passport, but her mac doesn't detect it.

I think it has to do with the fact the harddrive has USB 3.0 connection.

Any solutions?

USB 3.0 is supposed to be backward compatible with older versions. Could be some driver flap between Lion and the external drive, or that the drive is formatted with a different file system (read: Windows only) that Lion can't recognize.

Newer Macs (the June 2012 MacBooks (Pro & Air)) also come with USB 3.0 ports (shown as blue connectors). NTFS, which is what Windows uses by default, is also supported by Mac since 10.3, although with read-only support.
Can you check what format the hard drive is in? Most of the WD My Passports are formatted as NTFS by default now.
If you have another USB 3.0 cable, try that as well.

Does it show up in Disk Utility (search for it in Spotlight) when it's plugged in?

Going by the specs of My Passport, I think something's needed to mount the NTFS file system in use by that drive.

It's NTFS. I'll try to see if Disk Utility shows up on her mac.

OSX 10.7 has native NTFS read ability, but if you want to write to the device then you'll need to install some add-ons. I'd be suspicious of the USB cable. If the device isn't appearing in Disk Utility, try using another USB cable and see if it makes a difference. You can also try using another USB port on the computer (assuming she has more than one USB port).