All that needs to happen is that she needs to export the file to InDesign interchage format (inx) so you can open it. If she is editing any links at all she should also package it up afterwards and you should rename your links folder to force Indesign to use the new folder.
Be aware that going from version to version like this can cause problems so back up your files. Also text will likely reflow between the versions. It's not recommended to do this all the time for these reasons.
While Stix kindly gave you some instructions and gentle warning, I'll hit you a bit harder.
It's time for both of you to upgrade CS5.5 if you want to work together. Roundtripping through INX is just a bad, bad workflow. While you might get away with it for a single page, for a longer document you're going to be in for a world of headaches.