It may be that the file your friend found was an older sbf version than was already installed. If memory serves me correctly, if you use a 2.2 sbf, then try to flash a 2.1 sbf, you can get this error. Something about Motorola changing the keys when going to 2.2. I could only find
this one that seemed to be later than August 2010 (it's December 2010 - 2.2.1 [2.3.340]), but read up on it before using as I would hate for you to possibly make things worse (<< disclaimer). It supposedly will take you to stock, unrooted.