My quick research indicates that you should check Title 27 Article 4 Chapter 1.5 covering Auto Repair Claims Service of Indiana Statutes. You can also find the answer by contacting the state insurance commission or department of Indiana. Each state has its own insurance regulations regarding various issues. There is most definitely a regulation in Indiana which will answer that question. There is probably a person who deals with repair regulations at the State insurance commission who can easily answer your question. Usually a local reputable body shop will also have experience in dealing with this issue since it arising repeatedly. I don't think you need to hire an attorney and you could also speak to the adjuster handling the repairs and ask them to provide you with the authority which they claim permits them to use aftermarket parts.
I would check the above regulation as I think it will put you in the right direction. Good luck.
Disclaimer: The above is general legal infomation on a general topic and should not be considered legal advice. There is no attorney client relationship and none should be inferred or implied. You may always personally meet with a local attorney for legal advice.