it's from my exprience that all funky errors in integrating .Net with any micrsoft software (Active directory, exchange server, IIS, etc) are because the Hosting OS does not support the funcionality queried. The only OS i know to support these functionalities is Microsoft server 2003 WITH SP1.
it sounds strange, but i've seen it eneugh to know there's a decent chance that's the problem. Your "friend" should try to install SP1 on his dev server.
>However, they keep getting a "Cast is not valid" exception when trying out this line:
>cdoRecipient = (IMailRecipient) cdoPerson
try cdoPerson.GetType().ToString() to see what the type is.
Did you ever find an answer to this problem?
FYI: Similar to Tomer's comments, I have traditionally used ADSI. However, I havent needed to use CDOEXM for Mail-Enabled recipients, because so long as you set the appropriate attributes, the RUS will come visit the contact and add the other neccessary attribute annotations to the contact to finally mail-enable it. (not to be confused with Mailbox enabled, which does require CDOEXM, and requires a User not a Contact entity)