Get the Xbox to boot by using the Evo X cd. The FTP to it and check the contents of the c partition.
Look for the MSXBOXDASH.XBE AND XBOXDASH.XBE make sure that you can see both of them, if not then there is your problem. FTP the Evoxdash back to the Xbox along with your backup of the original xboxdash.xbe.
That should work.
If that makes no sence at all then maybe get your "buddy" to do it for you!