Just ask ur friend what he did to the phone!!:sly:
Usb debugging needs to be enabled.
Next thing is:- Stop worrying!! Flash the same firmware version(if u remembered exactly) which came preinstalled from factory. It may b froyo or gingerbread. Download that particular firmware & note that if u were having froyo preinstalled & have downloaded the same exact firmware, then it must be a 4 package firmware(i.e. containing 4 files when u uncompress it). U can use Odin 4.42 & follow this sequence (even if u know) :
*Install Samsung mobile drivers on pc.
*Take out & reinsert battery.
*Press "HOME,POWER,VOLUME-DOWN" buttons together for download mode.
*Launch Odin 4.42 as administrator.
* Connect the phone to pc & wait till u see COM PORT mapping box turned yellow.
* Now set the files from the froyo package that u downloaded in exactly the same sequence as follows:
Press start & wait till u see "PASSED"
Meanwhile ur phone will reboot.
Then it will b back to life(with froyo) guaranteed.
First try these steps!! I know it 100% works. coz I m used to all these issues.
Hope this helps...!!