Oh dear what a muppet he is...i hope you backed up your data??..if not then theres no way of getting it back...how can he forget a password that he put on there for a start??..kinda retarded if you ask me.
Anyhow...you need to download an OS for the 8900 curve..once you have done that then install it, once done then connect it to the desktop manager software that comes on the cd...once its connected...go to the app loader menu and click on it...it should load the OS up...just tick all the boxes and press next and it should install.
If you get stuck then head over to
www.crackberry.com where they have people there who can help you out and give you advice.
Basically in a nutshell you need to re-load the OS back onto the device.
In fact seeing as im in a helpful mood go here first to download the OS for your device:
http://forums.crackberry.com/f100/newest-8900-os-v4-6-1-250-multilanguage-245615/
Then go here to get instructions on how to reload it back onto your device...ignore the part about Memcrack and JL commander...you dont need to do those steps seeing as your 8900 is wiped clean:
http://forums.crackberry.com/f3/proper-upgrade-downgrade-procedure-53359/
Hope that helps and if you have any more problems i suggest you join up to crackberry and ask in there...they have guys in there that can answer most if not all your questions...just make sure you post in the right sub forum

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