Please keep in mind that if she isn't following your direction, you may need to replace her with a more BF friendly sitter. It's a slippery slope. Sadly in some situations, by the time the sitter has been educated, the damage to Mom and her confidence has been done. Just keep it in the back of your mind.
Take those articles the other ladies gave you to the sitter and ask her to read them. Then arrange for a meeting to discuss them. If she seems excited about them and is buying into them, then move forward. If she's arguing the point, her unwillingness may be a sign that she's not a good fit.