Upgrade iPhone 3GS to iOS6 with updated baseband

Hi all, For a friend I'm trying to update the iPhone 3GS to iOS6. Without knowing it was a jailbraken phone to avoid sim-lock, I just updated it through iTunes. Therefore I'm know stuck in the activation screen with installed iOS6 and I guess also the updated baseband. Restore via iTunes seems impossible since the message "we are unable to complete your activation" appears. In order to update iOS, I understood from link below that this is only possible 'without' updating the 'baseband' (whatever that may be... not so skilled with all this yet). http://www.iclarified.com/25144/how-to-upgrade-your-iphone-4-3gs-without-updating-the-baseband-windows So, question is whether it is still possible to jalbreak the phone to iOS6, even though the latest baseband has been installed? And is that even possible when I'm stuck in activation screen? If not, what are my options? Thanks a lot!!