What absolute NONSENS!!!
This guy obviously does not know a whole lot about Yorkies. I have 2 Yorkies girls (spayed) and 4 Biewers, Two boys and 2 girls ( Unspayed). My Lizzy (Biewer ) and Chadey (Yorkie ) who are not crazy about each, will now and again have their "I am mommies favourite" tiff, but nothing serious. I agree, if he dragged my puppy by the neck I would have hit.
Please get a new trainer.