your designer should save the file in idml format. than you can open that file with CS6
Do you have the CS6 version 8.0.2 update installed? If not, install it and try again.
It's also possible the other designer didn't really send you a .idml file, but just changed the file extension. That said, anyone with a CC subscription can download and install CS6, and that would be the preferred method for collaboration with someone using CS6.
Not being patched wouldn't prevent opening an IDML file. That restriction ended back with INX.
I vote for the changed extension instead of properly saving the file as IDML as the cause of this.