I've only met one person with an extremely small Yorkie, she was under three pounds at over a year (or maybe a year and a half). Anyways, she was totally cute and adorable, but she had a ton of health issues.
And when they had to take her to the vet, they got charged more because of her delicate nature and tinyness (not sure how "fair" that may be to charge more to treat smaller Yorkies, but I've heard it from several sources). She needed constant care, and I don't think they could really even leave her at home alone at all...
If your friend is dead set on getting an extremely small Yorkie, nothing we say may change her mind, just let her know to be prepared financially for any health problems and be prepared to spend a LOT of time with the Yorkie. And tell your friend good luck in finding the perfect Yorkie for her!