In two words...he cant.
You can only send via your ISP unless you are using a web based email service like hotmail.
He can enter his freeserve email account into the 'reply to email address' under tools/accounts/properties. it will then look like it came from his freeserve.
If he doesnt use the freeserve dial up, I think it will expire after so many days. Why doesnt he just get everyone to use his broadband email?