This is an age-old problem that exists in many facets of life. Who do you believe? Who do you trust? How do you know they are teaching the proper fundamentals? If you are getting your car repaired, how do you know the mechanic knows what he is doing? Are they taking advantage of you? I think you see where I am going with this.
This website, this forum, can be your saving grace. Here you are asking people you trust what you should do. You've already got a few to respond but the final decision for your course of action is yours. With that, I will share my 2 cents:
Unfortunately, the Better Business Bureau does not have a listing for pitching instructors but there are associations that certify pitching instructors Most notably for me is the NFCA (National Fastpitch Coaches Association) and the IFPA (International Fast Pitch Association) that are supposed to have a registry of members that have completed their courses on instruction. If a pitching instructor has taken the time to complete those courses and become certified and registered, I believe this goes a long way towards earning your trust. It should show their dedication to their education on the skill. Now, are they any good at what they teach? They could have all the certifications in the world and still be a dud. Go with your gut and go with an instructor that bonds with your daughter. If she's confused and frustrated, if the results are negative, I would go someplace else. Its one thing to have the knowledge but teaching it can be fickle. An instructor that would try to make major changes within the season for any other reason than personal injury may have an ego problem and that is definitely a red flag.
I have changed a girl's form in season because I had concerns she was going to hurt herself.
You may need to travel to find a good instructor. You may travel a long distance only to find another dud.
One of the things I do in real life, beyond following my gut, is to look for a winner for advice. If you are aware of a pitcher with a stellar track record, go ask her or her family who they go to. One thing about pitchers and their families is they love to share this kind of info. Talk with a local college and ask for advice. Most colleges will have their own pitching instructor but the pitchers came from instruction by somebody else. See if the have any local talent and ask them who they went to that got them there.
Don't go ask a loser. Why? It's name descriptive...........