Selecting where your replies come from

I have a user who is currently being forwarded e-mails from customers for a business he consults for. This results in clients contacting him at one address and his replies coming from another address. Is there a way to get outlook to allow him to choose what his return address should be so that it will appear that he is replying from his client’s server instead of his own? He has bandwidth issues logging onto his clients server, and doesn’t want to confuse their mutual customers by replying with his sub contractor address.