Without some sort of mailing list software, I'm not sure you'll really get the functionality you want.
As a workaround I would try setting up a custom tag, using an existing unused field, or something in Notes, and then creating a Smart Ground around that.
For example, all the contacts you want in your group, put 'mailing list' into their notes. Then create a SmartGroup to show those contacts.
Send the contacts to your friend, and they can import and merge the data. If they setup the same smart group, you should have the same contacts, with the same info, in that group.
The other option would be to stop her importing anything, and just use a Shared Addressbook, and maintain everything there. Google, Exchange, LDAP etc will let you use a remote address book, that isn't automatically synced to your local address book, but all contacts are accessibly, so you can both use that. In the groups page of the address book you'll see your local contacts, and remote contacts as distinct lists. Then, you'll just know to send to everyone in that shared address book.
It's possible to have more than 1 email account associated with an Apple ID, without the 2nd address actually being one. So for example while my Apple ID is from a personal domain, say xyz@1234.com, I also have an icloud email address xyz123@me.com that is associated with my Apple ID. If your wide used a mobile me account, maybe as part of a family pack, and you migrated it to icloud, it's possible that it has gained such a link in some way.
Best Answer
First check if the incorrect address is in Contacts. If so, you can just edit and correct it there.
If not, then it is probably stored in the address history. You can open this from the Window->Previous Recipients menu. You can search for the address in the dialog and delete it. (You might need to do this, even if you edit the email address in Contacts, as the wrong address might have been stored in the history.)