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To enable this, you would first need network (internet/LAN or other connectivity method) to connect your exchange server and your friend's office network.
Then you just need to allow the IP of frined's printer to allow him to relay (please keep it in mind if his outgoing email domain name is different than yours, your ISP may block your access as it is know as spamming).
The best way is, install IIS 7 on some workstation in your frined's office which will allow to send email out. http://technet.microsoft.com/en-us/library/cc772058(v=ws.10).aspx