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?