Although I've seen Morkies that look like Biewers, I believe the story that a Biewer is actually a cross between a Yorkshire Terrier and a Shih-Tzu. So mixing a Biewer again with a Yorkie to me would only increase its likelihood to look more like a yorkie.
Is your friend a breeder or just a pet owner who would keep the offspring?
-Although I understand the argument for purity, I think mixing happens and can be a good thing to some. The main issue is health. If the lines were healthy that's one thing but unknown backgrounds can be dangerous to the pup's quality of life.