For all intensive purposes Mesaba IS NW. There is no Mesaba ticketing system - just NW's - there is nothing wrong with the relationship. I'd try another call to CO, or a call to NW to see if its possible on their end before calling CO back (not possible for them to do it for you, since you need to do it with CO miles, but if they can do it IN GENERAL if someone was using NW miles)

Also, if your friends ticket is on Mesaba, its on a Saab 340 Turboprop, as that's the only plane Mesaba flies now - the ARJ's are gone (Avro Regional Jet with 16 FC), the CRJ's are Pinnacle, and Mesaba has the Saabs, the beautiful slow loud cramped saabs.