Sorry, there is not a way. AE has never been able to do it officially. There were hacks that worked sometimes in earlier versions, but not between AE 9 & AE 10. AE 9 is 32-bit, and AE 10 is 64-bit, a very fundamental difference.
You would need your buddy to replicate his AE 10 work in AE 9, or you could import rendered files from your buddy. But that's it.
How 'bout AE 10 & AE 10.5? You can backsave from 10.5 to 10 as long as the project contains none of 10.5's new features like Warp Stabilizer.