I've just tried to upgrade my boss's late 2010 and it was incredibly fussy about RAM. I tried 1600mhz modules and it would crash out during the installer every time.
Popped the 1333mhz stuff back in, works fine. There's reports of people popping the original RAM back in as needed but it seems like a total faff.
Are you looking for a 16gb kit perchance? If so, slim pickings at 1333mhz I'm afraid. Going to pop 8gb in his, will be more than ample.