Yes, that's actually what I used to do untill I installed Snow Leopard on my Windows laptop🙂
But you will have to run redsnow on windows and select "Just enter pwned DFU Mode right now" on your 3GS within your XP computer.
And then it will let you restore your iphone to the custom IPSW created with pwnage.
Or if you do the whole thing like create the custom IPSW and restore to it thru his itunes on his Mac then you dont have to do the rest I said above on your XP machine. Just sync and restore from your backup when you get home.