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Try it.
I already posted a full proof solution but I don't think you wanna follow it..

It's not that I don't want to, its that I don't know 100% on how to do it. How do I make the ipsw? Does ifaith work on a PC?

EDIT: Found my answer..
http://www.iphonejailbreakfaq.com/ifaith-save-shsh-blobs-of-old-firmware/

So run this program, dump SHSH. Restore to 4.2.8. Then within ifaith, run the "build signed ispw from blob", then redsn0w?

I thought itunes didn't have 4.2.8 though? If he jb'd the day he got the phone, what are the chances he'll have a backup of 4.2.8 saved? Especially if he's synced since?
 
A new ifaith is out. If u can wait I would ask u to wait little. Let some ppl test it then try it. Or u will b stuck with 4.2.10 :)

OK.. so wait to see that the latest ifaith works well, then run this program, dump SHSH. Restore to 4.2.8. Then within ifaith, run the "build signed ispw from blob", then redsn0w?
 
OK.. so wait to see that the latest ifaith works well, then run this program, dump SHSH. Restore to 4.2.8. Then within ifaith, run the "build signed ispw from blob", then redsn0w?

Wait.
See If it works.
Run ifaith dump shsh
Build a signed ipsw
Put phone In pwned dfu.
Shift restore In iTunes using the ipsw u made from ifaith
 
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