I'd love to know exactly which publishers these are who're getting mountains of shite in their slush pile every December. Has she actually talked to them, or is it just this one friend of hers telling her this?

I'd wager that EVERY publisher gets shite in their slush pile every December. Speaking as someone who gets literary submissions from the public, there are a lot of people out there trying to write things even without NaNoWriMo spurring them on -- and some of these people are irretrievably bad.

However, every publisher also gets shite year-round -- it's just the nature of the work. I wouldn't be surprised if there's a spike in the months after November, either. However, killing NaNoWriMo will not stem the flood of shite.