Hey SupahCoolX,
Success! I found the old mailboxes and reimported them and basically rebuilt Mail as my friend had it before (then he got the bright idea to ask me to import them into Entourage 'cause he "just wanted to try it out". It got ugly after that <heh heh>).
Looks like, when he got the bright idea to add a new account in Mail (based on a misinterpretation of advice of his ISP, which just partnered with Yahoo), he deleted the old one (and all its mailboxes). He _insists_ he did no such thing though (yah, right).
He owes me a great deal of beer for this one (I'll share with you :-)
Thanks again
Chas