I agree with the great advice you've gotten. Your best bet is to attack this in a very "psychological" way. A lot of people feed bad food. If someone berates them for their choice, they're not likely to change in their defensiveness. But if you act excited about your new-found information, she might listen. I guess it depends on how much you actually like her, but this can be a bonding experience. Encourage her to learn more about dogs with you. Encourage her to come visit us at Yorkie Talk or perhaps find her a Shih Tzu forum (just searched and found these)......
Shih Tzu Chatter Two Off The Leash ---- The Place To Hang Out for Shih Tzu Owners
Good Luck and I hope this turns out well and brings you together!!