I don't think it's possible to make a CS 5.5 project backwards-compatible with CS 5. Up until CS 6 (with its ability to save out CS 5.5 versions) AFAIK you've never been able to save an AE project to be compatible with an older version.
Could your client install the 30-day demo of AE CS 6 to get over this hump?
Marshall Arts Motion Graphics
You can save back from CS5.5 to CS5. Details are here:
If you don't have After Effects CS5.5, you can download the trial version and use it for this purpose. There's a handy set of direct links here: