The address for a reply comes from the message. If that is what you are seeing then your friend needs to fix their system.

If you are starting a message and typing their name in an address filed and the auto suggest is giving you the wrong address then you need to find the bad address in your address books and fix it. It is more than likely in your Collected book since you replied to the bogus address.

My original thought exactly, but he tells me that people who did not reply to the message are not seeing a bogus address in the reply line.

Check out your Collected address book and look for the bad address. Delete it or correct it.